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Investigation of buildup dose for therapeutic intensity modulated photon beams in radiation therapy

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Title:
Investigation of buildup dose for therapeutic intensity modulated photon beams in radiation therapy
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English
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Javedan, Khosrow
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University of South Florida
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Tampa, Fla
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Subjects / Keywords:
Skin Dose
Compensator-Based IMRT
Helical TomoTherapy
Monte Carlo Simulations
Chamber Measurements
Breast Cancer
Radiotherapy
Dosimetry
Buildup Dose
Step Jig
MLC-Based IMRT
Dissertations, Academic -- Chemical & Biomedical Engineering -- Masters -- USF   ( lcsh )
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non-fiction   ( marcgt )

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Abstract:
ABSTRACT: Buildup dose of Mega Voltage (MV) photon beams can be a limiting factor in intensity- modulated radiation therapy (IMRT) treatments. Excessive doses can cause patient discomfort and treatment interruptions, while underdosing may lead to local failure. Many factors which contribute to buildup dose, including the photon beam energy spectrum, scattered or contaminant radiation and their angular distribution, are not modeled well in commercial treatment planning systems. The accurate Monte Carlo method was employed in the studies to estimate the doses. Buildup dose of 6MV photon beams was investigated for three fundamentally different IMRT modalities: between Helical TomoTherapy and traditional opposed tangential beams, solid IMRT and multileaf collimator (MLC)-based IMRT techniques. Solid IMRT, as an alternative to MLC, achieves prescription dose distribution objectives, according to our study. Measurements and Monte Carlo calculations of buildup dose in chest wall treatment were compared between TomoTherapy IMRT and traditional tangential-beam technique. The effect of bolus in helical delivery was also investigated in this study. In addition, measurements and Monte Carlo calculations of buildup dose in solid IMRT and MLC based IMRT treatment modalities were compared. A brass step compensator was designed and built for the solid IMRT. Matching MLC step sequences were used for the MLC IMRT. This dissertation also presents the commissioning of a Monte Carlo code system, BEAMnrc, for a Varian Trilogy linear accelerator (LINAC) and the application in buildup dose calculation. Scattered dose components, MLC component dose and mean spectral energy for the IMRT treatment techniques were analyzed. The agreement between measured 6MV and calculated depth dose and beam profiles was (± 1% or ±1 mm) for 10x10 and 40x40 cm2 fields. The optimum electron beam energy and its radial distribution incident on tungsten target were found to be 6 MeV and 1 mm respectively. The helical delivery study concluded that buildup dose is higher with TomoTherapy compared to the opposed tangential technique in chest wall treatment. The solid and MLC IMRT comparison concluded that buildup dose was up to 7% lower for solid IMRT compared to MLC IMRT due to beam hardening of brass.
Thesis:
Dissertation (PHD)--University of South Florida, 2010.
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Includes bibliographical references.
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by Khosrow Javedan.
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Document formatted into pages; contains X pages.

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ABSTRACT: Buildup dose of Mega Voltage (MV) photon beams can be a limiting factor in intensity- modulated radiation therapy (IMRT) treatments. Excessive doses can cause patient discomfort and treatment interruptions, while underdosing may lead to local failure. Many factors which contribute to buildup dose, including the photon beam energy spectrum, scattered or contaminant radiation and their angular distribution, are not modeled well in commercial treatment planning systems. The accurate Monte Carlo method was employed in the studies to estimate the doses. Buildup dose of 6MV photon beams was investigated for three fundamentally different IMRT modalities: between Helical TomoTherapy and traditional opposed tangential beams, solid IMRT and multileaf collimator (MLC)-based IMRT techniques. Solid IMRT, as an alternative to MLC, achieves prescription dose distribution objectives, according to our study. Measurements and Monte Carlo calculations of buildup dose in chest wall treatment were compared between TomoTherapy IMRT and traditional tangential-beam technique. The effect of bolus in helical delivery was also investigated in this study. In addition, measurements and Monte Carlo calculations of buildup dose in solid IMRT and MLC based IMRT treatment modalities were compared. A brass step compensator was designed and built for the solid IMRT. Matching MLC step sequences were used for the MLC IMRT. This dissertation also presents the commissioning of a Monte Carlo code system, BEAMnrc, for a Varian Trilogy linear accelerator (LINAC) and the application in buildup dose calculation. Scattered dose components, MLC component dose and mean spectral energy for the IMRT treatment techniques were analyzed. The agreement between measured 6MV and calculated depth dose and beam profiles was ( 1% or 1 mm) for 10x10 and 40x40 cm2 fields. The optimum electron beam energy and its radial distribution incident on tungsten target were found to be 6 MeV and 1 mm respectively. The helical delivery study concluded that buildup dose is higher with TomoTherapy compared to the opposed tangential technique in chest wall treatment. The solid and MLC IMRT comparison concluded that buildup dose was up to 7% lower for solid IMRT compared to MLC IMRT due to beam hardening of brass.
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Compensator-Based IMRT
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Monte Carlo Simulations
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Dosimetry
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Step Jig
MLC-Based IMRT
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Investigation of Buildup Dose for Therapeutic Intensity Modulated Photon Beams in Radiation Therapy by Khosrow Javedan A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Chemical a nd Biomedical Engineering College of Engineering University of South Florida Co-Major Professor: William E. Lee III, Ph.D. Co-Major Professor: Harvey M. Greenberg, M.D. Geoffrey Zhang, Ph.D. Kenneth M. Forster, Ph.D. Kent H. Larsen, Ph.D. Paris H. Wiley, Ph.D. Date of Approval: July 14, 2010 Keywords: Skin Dose, Compensator-Based IMRT, Helical Tomo Therapy, Monte Carlo Simulations, Chamber Measurements, Breas t Cancer, Radiotherapy, Dosimetry, Buildup Dose, Step Jig, MLC-Based IMRT Copyright 2010, Khosrow Javedan

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DEDICATION To my family.

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ACKNOWLEDGEMENTS I would like to thank my advisors Geoffrey Zhang, Ph.D. and Kenneth Forster Ph.D. for giving me the opportunity to work on this project and guiding me throughout the completion of my graduate studies. The guida nce I was given has been invaluable. I am thankful to William E. Lee III, Ph.D. and Harvey Greenberg, M.D. for serving as comajor professors and for their encouraging support throughout this work. Dr. Lee has provided tremendous support and advice for my graduate career. I would like to thank Kent Larsen, Ph.D. and Paris Wiley, Ph.D. for serving on my supervisory committee and for their encouraging support. I also would like to express deep gratitude to the following: my current employer H. Lee Moffitt Cancer Center & Research Institute, for providing the environment and resources for this work; Craig Stevens, M.D., Ph.D., for his continued encouragement and support for this work; Stuart Wasserman for his support; William Totten, for his effort and technical support in Linux OS and aid in installing the EGSnrc and BEAMnrc Monte Carlo Code Systems on the Moffitt Computer cluster; and Varian Medical Systems for providing the Monte Carlo data package for our high energy Accelerator; and Richard Sweat, Ken Cashon, Chris Warner, Lisa Cas hon and everyone at Dot Decimal Company, for their technical support and comp ensator material for this work.

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Above all, I cannot thank enough my famil y, my wife Michele and my son Anthony Reza for their understanding and support and positive encouragement throughout my graduate studies.

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i TABLE OF CONTENTS LIST OF TABLES ............................................................................................................. iv LIST OF FIGURES .............................................................................................................v ABSTRACT ..................................................................................................................... viii CHAPTER 1 INTRODUCTION ........................................................................................1 1.1 Synopsis ............................................................................................................1 1.2 Objective of the Study ......................................................................................2 1.3 Dissertation Outline ..........................................................................................3 1.4 Limitation of this Work ....................................................................................5 CHAPTER 2 MONTE CARLO SIMULATION ................................................................6 2.1 Synopsis ............................................................................................................6 2.2 Monte Carlo Simulation of Megavoltage Photon Beam ..................................6 2.3 Material and Methods .......................................................................................8 2.3.1 Monte Carlo Simulation of the Varian Clinac 6MV Beam ..............8 2.3.2 Component Modules of BEAMnrc ...................................................8 2.3.3 CMs for Varian Clinac 2100 Model .................................................9 2.3.4 DOSXYZnrc ...................................................................................10 2.3.5 Measured Beam Data ......................................................................11 2.3.6 Accelerator simulation Parameters .................................................11 2.3.7 Phase Space File .............................................................................13 2.3.8 Computer Cluster for MC Simulation.............................................15 2.4 Results .............................................................................................................15 2.4.1 Comparison Between Measured and Calculated Percent-DepthDose (PDD) Curves ...................................................................................15 2.4.2 Comparison Between Measured and Calculated Beam Profiles .....19 2.4.2.1 10x10 cm2 Beam Profile ...................................................19 2.4.2.2 40x40 cm2 Beam Profile ...................................................19 2.4.3 6MV Spectrum and Fluence ...........................................................29 2.5 Conclusions .....................................................................................................34 CHAPTER 3 PAPER I: SKIN DOSE STUDY OF CHEST WALL TREATMENT WITH TOMOTHERAPY ..................................................................................................35 3.1 Synopsis ..........................................................................................................35 3.2 Introduction .....................................................................................................36

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ii 3.3 Material and Methods .....................................................................................38 3.3.1 Patient Cases ....................................................................................38 3.3.2 TomoTherapy Planning ...................................................................39 3.3.3 Tangential-Beam Planning...............................................................40 3.3.4 Monte Carlo Simulation ...................................................................41 3.3.5 Dose Measurements .........................................................................42 3.4 Results .............................................................................................................44 3.4.1 Film Dosimetry ................................................................................44 3.4.2 MOSFET Dose Measurement ..........................................................47 3.4.3 Monte Carlo Study ...........................................................................48 3.4.4 Plan Analysis and Comparison ........................................................49 3.4.4.1 Discussion .........................................................................51 3.5 Conclusion ......................................................................................................52 CHAPTER 4 PAPER II: COMPENSATOR-BASED INTENSITYMODULATED RADIATION THERAP Y FOR MALIGNANT PLEURAL MESOTHELIOMA POST-EXTRA PLEURAL PNEUMONECTOMY ..........................53 4.1 Synopsis ..........................................................................................................53 4.2 Introduction .....................................................................................................55 4.3 Materials and Methods ....................................................................................58 4.3.1 Surgery .............................................................................................58 4.3.2 Simulation ........................................................................................59 4.3.3 Contours ...........................................................................................59 4.3.4 Treatment Planning ..........................................................................60 4.3.5 IMRT Plans ......................................................................................60 4.3.6 IMRT Prescription Page ..................................................................61 4.3.7 Compensator Plans...........................................................................62 4.3.8 Treatment Planning Strategy ............................................................62 4.3.9 Compensator Thickness File ............................................................64 4.4 Safety Considerations .....................................................................................65 4.4.1 Plan Evaluation ................................................................................66 4.4.2 Quality Assurance ............................................................................67 4.5 Results .............................................................................................................67 4.5.1 QA Results .......................................................................................72 4.6 Discussion .......................................................................................................74 4.7 Conclusion ......................................................................................................75 CHAPTER 5 PAPER III: 6MV BUILDUP DOSE FOR COMPENSATORBASED IMRT COMPARED TO MLC-BASED IMRT ...................................................77 5.1 Synopsis ..........................................................................................................78 5.2 Introduction .....................................................................................................78 5.3 Material and Methods .....................................................................................82 5.3.1 Study Setup ......................................................................................82 5.3.2 Solid Brass Modulator .....................................................................83 5.3.3 MLC Step and Shoot Sequences ......................................................84

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iii 5.3.4 Matching Profiles at 10 cm Depth ...................................................85 5.3.5 Chamber Measurements in the Buildup Region ..............................86 5.3.6 Monte Carlo Modeling .....................................................................87 5.3.7 Dose in Buildup Versus Source Surface Distance ...........................89 5.4 Results .............................................................................................................90 5.4.1 Dose Profile Match at 10 cm Depth .................................................90 5.4.2 Dose Comparison at Shallow Depths ..............................................91 5.4.3 Shallow Dose Variation with SSD ...................................................93 5.4.4 Shallow Dose Variation with Field Size ..........................................94 5.4.5 Energy Spectra Variation for Open and Compensated Field ...........95 5.4.6 Dose Contributions of Various Components ...................................97 5.4.7 Scatter Photon Dose Contribution ...................................................98 5.4.8 Contaminant Electron Dose Contribution ........................................99 5.4.9 MLC Component Dose Contribution ...............................................99 5.5 Discussion .......................................................................................................99 5.5.1 Measured Dose Gradient ................................................................102 5.6 Conclusion ....................................................................................................102 CHAPTER 6 CONCLUDING REMARKS ....................................................................104 6.1 Recommendations for Future Work ..............................................................104 REFERENCES ................................................................................................................106 ABOUT THE AUTHOR ....................................................................................... End Page

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iv LIST OF TABLES Table 1. Comparison of the measured superficial dose between TomoTherapy and tangential-beam techniques on a Rando phantom using MOSFET. ...........47 Table 2. Normalized dose at the surface, 2 and 5 mm depth is compared for chest wall treatment plans using the tangential-beam technique and TomoTherapy. ...................................................................................................50 Table 3. Dose–volume guidelines for th e target and organs at risk (OARs)a. ................62 Table 4. Plan valuesa. ......................................................................................................71 Table 5. Plan delivery values a. .......................................................................................72 Table 6. Doses (cGy) of IMRT delivery with solid modulator and MLC as a function of STEP thickness of the compensator at 1 mm depth. ......................91 Table 7. Doses (cGy) of IMRT delivery with solid modulator and MLC as a function of STEP thickness of the compensator at 3 mm depth. .......................92 Table 8. Doses (cGy) of IMRT delivery with solid modulator and MLC as a function of STEP thickness of the compensator at 5 mm depth. ......................92 Table 9. Monte Carlo percent of to tal dose contribution from scattered photons, contaminant electrons and MLC. ......................................................................97

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v LIST OF FIGURES Figure 1. The accelerator model for the 6MV Varian 2100 and its component modules in (a) XZ view, and (b) YZ view. ......................................................10 Figure 2. Source model as parallel circ ular beam with a uniform distribution. ..............12 Figure 3. Overlay of measured 6MV phot on depth dose curves (solid line) and Monte Carlo (circle) for 100 cm SSD and field size (a) 10x10 cm2, and (b) 40x40 cm2 calculated with 5.70 MeV elect ron beam incident on the target. ...............................................................................................................16 Figure 4. Overlay of measured 6MV phot on depth dose curves (solid line) and Monte Carlo (circle) for 100 cm SSD and field size (a) 10x10 cm2, and (b) 40x40 cm2 calculated with 6 MeV electron beam incident on the target. ...............................................................................................................17 Figure 5. Overlay of measured 6MV phot on depth dose curves (solid line) and Monte Carlo (circle) for 100 cm SSD and field size (a) 10x10 cm2, and (b) 40x40 cm2 calculated with 6.30 MeV elect ron beam incident on the target. ...............................................................................................................18 Figure 6. Overlay of measured 6MV photon beam profile (solid line) and Monte Carlo (circle) for 10x10 and 40x40 cm2 fields at 100 cm SSD and at depths dmax (a, b), 5 cm (c, d) and 10 cm (e, f) in water, with simulated electron beam energy inci dent on the target was 5.70 MeV ...........20 Figure 7. Overlay of measured 6MV photon beam profile (solid line) and Monte Carlo (circle) for 10x10 and 40x40 cm2 fields at 100 cm SSD and at depths dmax (a, b), 5 cm (c, d) and 10 cm (e, f) in water, with simulated electron beam energy incident on the target was 6.0 MeV.. ...........23 Figure 8. Overlay of measured 6MV photon beam profile (solid line) and Monte Carlo (circle) for 10x10 and 40x40 cm2 fields at 100 cm SSD and at depths dmax (a, b), 5 cm (c, d) and 10 cm (e, f) in water, with simulated electron beam energy incident on the target was 6.30 MeV. ..........26 Figure 9. Calculated photon spectrum in the form of planar fluence histogram for the region 0 r 3 cm inside a 10x10 cm2 field at 100 cm SSD. .....................30

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vi Figure 10. Calculated energy fluence distribution for 10x10 cm2 field. ...........................31 Figure 11. Calculated mean ener gy distribution ac ross (a) 10x10 cm2 field and (b) 40x40 cm2 field. ...............................................................................................32 Figure 12. Calculated fluence versus position for (a) 10x10 cm2 field and (b) 40x40 cm2 field. ...............................................................................................33 Figure 13. Calculated energy fluence versus position for 40x40 cm2 field. ......................33 Figure 14. The TomoTherapy chest wa ll treatment plans on a male Rando phantom............................................................................................................40 Figure 15. Film and MOSFET dose meas urement setup with TomoTherapy cheese phantom. ...............................................................................................43 Figure 16. Dose gradient difference between the film measurement and the treatment plan...................................................................................................44 Figure 17. (A) Monte Carlo calculated TomoTherapy percentage depth dose (PDD) versus measured PDD.. ........................................................................46 Figure 18. Skin dose comparison of To moTherapy with tangential beam technique. .........................................................................................................50 Figure 19. Two modulators from Decimal mounted on the Siemens coded trays. ........64 Figure 20. Dose distributions and profiles in the coronal and sagittal planes. ..................68 Figure 21. Dose–volume histograms for the planning target volume (PTV), clinical target volume (CTV), and the liver, lung, kidneys, and spinal cord for (a) a right-sided case, and (b) the left -sided case. ..............................70 Figure 22. The calculated and measured isodos e distributions in the coronal plane for one of the compensator fields is shown in the top right and top left quadrants. .........................................................................................................73 Figure 23. Brass modulator mounted on an ope n port Plexiglas tray inserted into upper wedge slot of LINAC. ............................................................................83 Figure 24. Monte Carlo model of Vari an accelerator head geometry. ..............................88 Figure 25. The matched profiles of IMRT de livery with step and shoot and solid modulator. ........................................................................................................90

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vii Figure 26. Percent of dose difference for IMRT test delivery with MLC with respect to compensator is shown as a function of step modulation (thickness) for 1, 3 and 5 mm depth. ..............................................................93 Figure 27. Buildup dose variation with SSD for open and compensated fields. ...............94 Figure 28. Dose variatio n with field size. ..........................................................................95 Figure 29. Normalized planar energy flue nce distribution of 6MV beam for 2x15 cm2 field. ..........................................................................................................96 Figure 30. Most probable compensator th ickness from 50 retrospective IMRT field analysis (Opp et al.). ..............................................................................101

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viii Investigation of Buildup Dose for Therapeutic Intensity Modulated Photon Beams in Radiation Therapy Khosrow Javedan ABSTRACT Buildup dose of Mega Voltage (MV) photon beam s can be a limiting factor in intensitymodulated radiation therapy (IMRT) treatments. Excessive doses can cause patient discomfort and treatment interruptions, wh ile underdosing may lead to local failure. Many factors which contribu te to buildup dose, includi ng the photon beam energy spectrum, scattered or contaminant radiat ion and their angular distribution, are not modeled well in commercial treatment pla nning systems. The accurate Monte Carlo method was employed in the studies to estimate the doses. Buildup dose of 6MV photon beams was investig ated for three fundamentally different IMRT modalities: between Helical TomoTh erapy and traditional opposed tangential beams, solid IMRT and multileaf collimator (MLC)-based IMRT techniques. Solid IMRT, as an alternative to MLC, achieve s prescription dose dist ribution objectives, according to our study.

PAGE 13

ix Measurements and Monte Carlo calculations of buildup dose in chest wall treatment were compared between TomoTherapy IMRT and tr aditional tangential-beam technique. The effect of bolus in helical delivery wa s also investigated in this study. In addition, measurements and Monte Carlo calculations of buildup dose in solid IMRT and MLC based IMRT treatment modalities we re compared. A brass step compensator was designed and built for the solid IMRT. Matching MLC step sequences were used for the MLC IMRT. This dissertation also presents the comm issioning of a Monte Carlo code system, BEAMnrc, for a Varian Trilogy linear acce lerator (LINAC) and the application in buildup dose calculation. Scattered dose com ponents, MLC component dose and mean spectral energy for the IMRT treatment techniques were analyzed. The agreement between measured 6MV and cal culated depth dose and beam profiles was ( 1% or 1 mm) for 10x10 and 40x40 cm2 fields. The optimum electron beam energy and its radial distribution inci dent on tungsten target were found to be 6 MeV and 1 mm respectively. The helical delivery study concluded that buildup dose is higher with TomoTherapy compared to the opposed tangential techniqu e in chest wall treatm ent. The solid and MLC IMRT comparison concluded that buildup dose was up to 7% lower for solid IMRT compared to MLC IMRT due to beam hardening of brass.

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1 CHAPTER 1 INTRODUCTION 1.1 Synopsis Determination of buildup dose of therapeu tic megavoltage photon beams has been an active area of research in radiation therapy si nce before the introduc tion of 3D conformal radiation therapy(1), a precursor to IMRT(2). Accurate knowledge of buildup dose of IMRT is necessary, especially for IMRT cases treated with concurre nt radiosentisising chemotherapy where excessive dose in the buildup region can cause skin infection and treatment interruption, and under dosing may lead to local failu re. Dose in this region must accurately be known so that the calcu lated dose by the treatment planning system (TPS) is properly interpreted. A radiation oncologist may have to compromise a known therapeutic dose in order to limit the skin dose calculated by th e treatment planning system (TPS). Historically, superficial dose is not well predicted by commercial TPS. Literature shows TPS overestimate dose in the buildup region by up to 19%. Literature also shows the expected calculation accuracy for pass fail criteria in the buildup region when commissioning the TPS is 20% of the normalization dose for open fields(3). It is up to the individual physicist to ac curately assess the shallow dose and incorporate that into evaluating the TPS dose in the buildup region.

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2 Many factors contribute to buildup dose, in cluding the photon be am energy spectrum, contaminant electrons and scattered partic le angular distributi on, and effect of immobilization devices which are not pr operly modeled in commercial treatment planning systems. The dosimetrical differen ces in buildup regions between different treatment modalities, such as helical IMRT de livery with TomoThera py versus traditional wedge pair technique, and MLC-based vers us compensator-based IMRT, cannot be accurately obtained by comparing treatment plans alone. This dissertation work inves tigates the dosimetrical differe nces in buildup region between TomoTherapy versus conventional wedge pair technique with and without bolus, and IMRT with MLC versus solid brass compensator with measurement and Monte Carlo method. Significant work was carried out in establishing and running the Monte Carlo Code system on the Moffitt Computer Clus ter and in commissioning the BEAM to perform radiation transport calc ulations with the same beam characteristics as the 6MV Varian Clinac 2100 beam. Use of this code was an essential part of this dissertation. 1.2 Objective of the Study The objective of the study was to investigate the dosimetrical differences including the dose and dose gradient in the buildup region of therapeutic photon beams from 3 different IMRT modalities: TomoTherapy, compensato r-based IMRT and MLC-based IMRT. The dose and relative dose distributions were m easured with film, ion chamber and MOSFET detectors and calculated with Monte Carlo to verify the doses. The results of the study

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3 were used to answer important clinical conc erns related to the IM RT technique used. For example, whether chest wall treatment with TomoTherapy requires the use of bolus material, whether solid IMRT can achieve the prescription dose distribution objectives as an alternative to MLC-base d IMRT, and whether buildup do se of IMRT delivery with compensator on the Varian Clinac 2100 LINAC is a concern. The secondary objective and essent ial part of this dissertatio n was to install the EGSnrc, BEAMnrc Monte Carlo code system on the in stitution’s computer cluster and test to ensure proper functionality of the instal led code and related programs, such as DOSXYZnrc, statdose, BEAM_DP and ct create. The Monte Carlo BEAM was commissioned to perform radiation tran sport calculations with the same beam characteristics as the 6MV beam of the LINAC used. 1.3 Dissertation Outline The European format of using peer-reviewed jo urnal articles in compiling the bulk of this manuscript has been adopted for this disser tation. Therefore, there may be overlapping text in various chapters of this work. This was ascertained to be the most efficient arrangement in order to preserve the overall quality of this work. Chapter 2 describes the commissioning of the Monte Carlo simulation code BEAMnrc and DOSXYZnrc for the application in the dissertation studies.

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4 Chapter 3 discusses the skin dose differences between TomoTherapy chest wall irradiation and traditional linear-accelerator -based tangential-beam technique. Adequate treatment of the chest wall using the tangential-beam technique is reviewed. Chest wall plans that were generated using two comm ercial treatment planning systems which produce plans for fundamentally different dose delivery methods, along with Monte Carlo dose calculations were ev aluated to determine if bolus was required for adequate skin dose from the two treatment techniques. Chapter 4 investigates the use of solid brass modulators for intensity-modulated radiation therapy (IMRT) delivery of large targets as an alternative to step and shoot delivery with multileaf collimator (MLC)(15). This study was conducted duri ng the initial use of solid modulators in the department to investigate th e device’s ability to reproduce the planned isodose distribution for a large target which overlapped norm al critical structures. An ideal modulator is one which faithfully repr oduces the field’s id eal intensity map as planned, both dosimetrically and spatially. The dose volume histogram (DVH) of IMRT plans with solid modulator and MLC was co mpared. The absolute point doses were measured with a calibrated ionization cham ber. The relative dose distribution was measured with EDR2 film and a commercial diode array device to ensure the planned isodoses matched the delive red isodose distributions. Chapter 5 investigates the dosimetrical di fferences in buildup region of a 6MV beam between MLCand compensator-based IMRT with measurement and Monte Carlo method. Photon beam energy spectrum, contam inant electrons and scattered particle

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5 angular distribution affect the buildup dose a nd are not properly modeled in commercial treatment planning systems. Literature sugge sts such systems overestimate the dose in this region. Since buildup dose near skin is not accurately predicted by commercial treatment planning systems, accurate Monte Carlo was used to calculate the near skin buildup dose at depth of 1.0-5.0 mm. Skin dose variation with SSD, field size and beam incidence angle was investig ated. Component doses of contaminant photons, contaminant electrons and MLC component dose was calculated for the two IMRT delivery systems. Mean spectral energy as a function of bra ss modulation was calculated to show beam hardening effect responsible for enhanced skin sparing of the solid modulator. 1.4 Limitation of this Work The Monte Carlo simulation package BEAM nrc was commissioned for the 6MV photon beam from Varian Trilogy LINAC equipped with MLC. However, Varian Trilogy machine also produces 15MV photon beams and 6 electron beams of 6 MeV to 22 MeV (commissioning these energies is left for futu re work). Due to simplified design of the brass step jig, we could ac curately model the steps of solid modulator and dose equivalent MLC sequences to perform simulation and dose and component dose calculation in the buildup region. Clinical IM RT beams may require highly complex dose distribution in X, Y and Z di rection, modulated with soli d brass or MLC. The modeling of such a complex device with BEAMnrc was out of scope of this research.

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6 CHAPTER 2 MONTE CARLO SIMULATION 2.1 Synopsis Monte Carlo simulation is a numerical solution to a problem that is not easily solved by analytical methods. The problem models objects (i.e, high energy electron and photon radiation) that interact with other objects (i.e, matter) in a well defined environment. A solution (result of interacti on based on actual radiation tran sport physics) is determined by repeated random sampling using computati onal algorithms to calculate the result. Monte Carlo simulations are employed in many fields such as radiation physics, chemistry, space, finance, mathematics, a nd other disciplines that may require a quantitative solution to a problem which can be approximated by statistical sampling. Monte Carlo techniques for simulating radia tion transport of elect rons and photons are used extensively in the field of medical physics and radiation dosimetry(4). Monte Carlo simulation has been found to be the most accurate technique to estimate the dose deposited in tissue based on the ac tual radiation transport physics(5, 6). 2.2 Monte Carlo Simulation of Megavoltage Photon Beam Monte Carlo simulation starts as the monoene rgetic electron beam with initial kinetic energy Eki interacts with the nuclei of the high Z tungsten target atom mostly by the way

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7 of Coulomb interacti on. The incident elect rons also scatter a nd lose energy through production of x-ray bremsstrahlung photons. Br emsstrahlung photons are produced at a rate expressed by the mass ra diative stopping power (dT/ dx)r in units of MeV cm2/g as in equation 1, 2 2 21 c m A Z dx dTo r (1) Z2/A refers to the atomic number and mass number of the medium and (moc2) is the rest energy of the charged particle. Bremsstrahlung is produced at higher rate for the high Z target compared to low Z target due to the Z2 dependence. Each particle is transported and tracked as it passes throug h and interacts with various components in the accelerator head in its path. Such compone nts are the primary collimator (defines radiation port), thin vaccum window (target assembly kept in vacuum), flattening filter (has cone shaped geometry, made of low-medium Z material, reduces the forward peaked photon fluence more in the center than periphery to produce flat beam), transmission chamber (monitors dose, beam flatness and symmetry), mirror (locked in place in x-ray mode to project fi eld light on surface), s econdary jaws (high Z material defines field size), ML C (high Z material shapes fi elds with motorized leaves) and the intervening air until it reaches the end of its history. It then either escapes the defined geometry or deposits its en ergy at the end of its track.

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8 The end of each particle’s track is determined by its cut o ff energy. Once particle energy falls below its cut off energy, the particle is no longer tracked and al l of its energy is deposited at the site, notwithstanding that Co mpton scattering is the predominant mode of interaction for the 6MV photon beam, interac ting with low Z abso rbers such as the flattening filter materi al, muscle and water. 2.3 Materials and Methods 2.3.1 Monte Carlo Simulation of the Varian Clinac 6MV Beam An EGSnrc(7, 8)-based Monte Carlo simulation packag e for clinical radiation treatment units, BEAMnrc (9), was used to precisely characterize a Varian Clinac 2100 LINAC (Varian Medical Systems, Palo Alto, CA) equipped with MLC. The detailed drawing including the materi al and geometry for the accelerator head components and the distance of each component from the target was acquired from the manufacturer. The component modules (CMs) of BEAMnrc were used to precisely model the accelerator head components in the accelerator input file. 2.3.2 Component Modules of BEAMnrc Component modules are blocks with front surface and back surface. All blocks are completely independent. Various CMs are used to model exact geometry and material of

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9 different components in the LINAC head. Ea ch component assumes a horizontal slab portion of the accelerator with respect to beam axis. Each specializ ed CM can be used more than once to model different parts of the accelerator, therefore a unique name was given to each component’s CM for the simulation. 2.3.3 CMs for Varian Clinac 2100 Model The CMs that were used to model the Va rian Clinac 2100 were SLABS for target, CONS3R for primary collimator, SLABS for vacuum window, FLATFIL for flattening filter, CHAMBER for transmission monitor chamber, MIRROR for mirror, JAWS for secondary collimator jaws, SLABS for air gap, and CHAMBER as phantom for phantom defined at 100 cm SSD (source to surface dist ance). Phantom in BEAMnrc input file was used to simulate depth dose in water along th e central-axis. The MLC was modeled using VARMLC CM in BEAMnrc. This module models the leaves, the air gap between leav es, the leaf tongue-in -groove and the driving screws at the top and bottom of each leaf. The accelerator model that was used to perf orm MC simulations is shown in Figure 1.

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10 Figure 1. The accelerator model for the 6MV Varian 2100 and its component modules in (a) XZ view, and (b) YZ view. The accelera tor components are th e target (1), the primary collimator (2), the vacuum window (3), the flattening filter (4), the transmission chamber (5), the secondary jaws (6) and the MLC (7), as shown in the figure. 2.3.4 DOSXYZnrc DOSXYZnrc(13) is another Monte Carlo simulation pr ogram which is used to calculate dose distribution in a simple rectilinear phantom geometry. The phantom is defined within the program by the user. For example a 40x40x40 cm3 water phantom was defined. Also the voxel size in X, Y, and Z dimension was input and dose distribution calculation planes either parallel or normal to the beam axis were defined. Percent depth dose and beam profiles were cal culated using this program.

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11 2.3.5 Measured Beam Data Beam data including the percent depth dose and beam profiles were acquired during the commissioning of the LINAC. Scanned beam da ta were measured with ion chamber and diode using a commercial water scanning system (Scanditronix-Welhoffer RFA 300). 2.3.6 Accelerator Simulation Parameters Three electron beam incident energies of 5.7, 6.0 and 6.3 MeV were investigated. The calculated percentage depth dose and beam profiles were compared with the 6MV scanned beam data to determine the optimum value for the electron beam energy incident on the target. The incident electron beam source was chosen as parallel circular beam with a uniform distribution (ISOURCC=1), as show n in Figure 2. In the accelerator input file, this source model takes four input parameters, the beam radius and the (x, y, z)-direction cosines. The xand y-axis direction cosi ne was set to zero and the zaxis direction cosine was set to 1 to define the incident beam orientati on parallel to the z-axis and pointing down the accelerator.

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12 Figure 2. Source model as parallel circul ar beam with a uniform distribution. (ISOURC=1)(9) is shown. The beam radius and the x,y,z -direction cosines are the 4 parameters used to define the source. The para llel circular beam is always assumed to be incident on the center of front of the firs t component module (i.e. at Z_min_CM(1)). Based on the information in the literature (10-12), the parameters of the primary electron beam incident on the high Z target, including its energy and beam radius, were chosen to closely match the simulated beam profiles an d percentage depth dose curves with the measured beam data. Simulation parameters used for BEAMnr c and DOSXYZnrc were the Global cut off energy: for electrons ECUT= 0.70 MeV a nd for photons PCUT = 0.01 MeV. Electron range rejection and photon forcing was turn ed off for all simulations. Electron range rejection is used to save computing time dur ing simulations where the range of charged particle is calculated and its history is term inated if it cannot leav e the region it is in.

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13 Photon forcing, where users fo rce the photon to interact in a specific CM, improves statistics. The cross section data for all material densitie s in the accelerator for particles with kinetic energy down to 10 KeV were from PE GS4 data file 700icru.pegs4dat. Selective bremsstrahlung split ting was turned on to save simulation time. The minimum and maximum number of bremsstrahlung photons produced by each bremsstrahlung event was set to 20 and 200 brems photons. The effective field size in which selective bremsstrahlung splitting probabilities are calculated was set to 30 for the 10x10 cm2 field and 50 for the 40x40 cm2 field respectively. This techni que improved the simulation time by a factor of 6 compared to no variance reduction. Typically between 108-109 histories are needed depending on the pixel size in the X, Y and Z dimension in order to yield a statistic ally acceptable solution, as fewer numbers of particles interact in smaller volume. 2.3.7 Phase Space File Phase space file is one of the most important outputs of BEAMnrc. Phase space file is a binary file and is usually tens of gigabyt es of RAM in size. Information about each particle history including its charge, energy, position, directi on of incidence and latch is stored in the phase space file. BEAMnrc outpu ts phase space file fo r planes that were

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14 scored at the end of each component module, CM. The user specifies the desired plane for phase space file to be scored by making the selection in the BEAMnrc input file. The phase space file can be used as a radiatio n source for further simulations in order to save time and hard disk space. For example, pa rticles can be collected in the phase space file at the end of secondary ja ws. The phase space file is then used as radiation source to simulate varying field sizes defined by MLC which is below the jaws (which is now the first CM as opposed to the target) or even downstream further away from the jaws where varying thickness compensators are simulated without having to resimulate the entire accelerator for each field size or compensator thickness, thus significantly speeding up the simulation. The BEAM data processor BEAM-dp (14) was used to analyze the phase space files. The program was accessed using graphical user interface gui, beamdp_gui command in the accelerator directory. The progr am outputs the requested info rmation such as fluence versus position, energy fluence versus positi on, energy fluence distribution, mean energy distribution, and angular dist ribution of the simulated elec trons and photons in the phase space file which can be plotted for visual analysis. The 6MV photon spectrum, the energy fluence distribution, mean energy distribution versus off axis distance for 10x10 and 40x40 cm2 fields, fluence and energy fluence across the 10x10 cm2 and 40x40 cm2 fields, and the photon spectra were calculated.

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15 2.3.8 Computer Cluster for MC Simulation A Rocks 5.2 cluster of 22 computers has be en used to accept independent batch jobs submitted by the MC code to the Q and dist ributed to local nodes by the master. The cluster runs Cent OS 5.3 dist ribution and is configured with 22 local nodes. One master server distributes the jobs to local nodes. Each compute node has two Xeon X5460 (3.2 GHz) Quad core processor with 32 GB me mory. Compute nodes are interconnected by a private switch at 1 Gbps. Each compute node can handle simulation calculations independently of other nodes, therefore as many as 176 independent simulation jobs can be run simultaneously. 2.4 Results 2.4.1 Comparison Between Measured and Calculated Percent-Depth-Dose (PDD) Curves Good agreement ( 1% or 1 mm) is seen be tween calculated PDD curves (circle) for all beam energies (5.7, 6, 6.3MeV) and measured 6MV PDD curves (solid line) for (a) 10x10 cm2 and (b) 40x40 cm2 fields in the buildup to 5 cm depth range as in Figures 3 (a,b), 4 (a,b) and 5(a,b). The calculated PDD curves for 5.7 MeV beam were 1-2% lower than measured 6MV PDD curve beyond 5 cm depth for 10x10 and 40x40 cm2 fields as in Figure 3 (a,b).

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16 (a) (b) Figure 3. Overlay of measured 6MV photon de pth dose curves (solid line) and Monte Carlo (circle) for 100 cm SSD and field size (a) 10x10 cm2, and (b) 40x40 cm2 calculated with 5.70 MeV electron beam incident on the target.

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17 (a) (b) Figure 4. Overlay of measured 6MV photon de pth dose curves (solid line) and Monte Carlo (circle) for 100 cm SSD and field size (a) 10x10 cm2, and (b) 40x40 cm2 calculated with 6.0 MeV electron beam incident on the target.

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18 (a) (b) Figure 5. Overlay of measured 6MV photon de pth dose curves (solid line) and Monte Carlo (circle) for 100 cm SSD and field size (a) 10x10 cm2, and (b) 40x40 cm2 calculated with 6.30 MeV electron beam incident on the target.

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19 2.4.2 Comparison Between Measured and Calculated Beam Profiles 2.4.2.1 10x10 cm2 Beam Profile For 10x10 cm2 fields, good agreement is seen betw een the measured 6MV beam profiles (solid line) and calculated (circle) beam profile s of the same fields at dmax, 5 cm and 10 cm depth for beam energies of 5.7 MeV as in Figure 6 (a-f), 6.0 MeV in Figure 7 (a-f), and 6.3 MeV in Figure 8 (a-f) respectively. The agreement between measured and calculated beam profiles for 10x10 cm2 beam at dmax, 5 and 10 cm depth was ( 1% or 1 mm) as in Figures 6 (a,c), 7 (a,c,e) and 8 (a,c, e).for all beam energies and depths except 5.7 MeV beam profile at 10 cm depth was 12% lower than that measured as in Figure 6(e) There was also a small increase in the size of the horn at dmax for the 5.7 MeV beam, as in Figure 6(a). 2.4.2.2 40x40 cm2 Beam Profile For the 40x40 cm2 fields, there was good agreement ( 1% or 1 mm) between measured 6MV beam profiles and calculated beam profiles of 6.0 MeV and 6.3MeV beams for most depths, as in Figures 7 (b,d,f) and 8 ( d,f), except 6.3 MeV beam profile at dmax was 1-2% lower than measured as in Figure 8 (b). The calculated 5.7 MeV beam profile, however, exhibited +6% increase in the size of the horn at all depths compared with the 6MV measured beam profile for 40x40 cm2 field, as in Figure 6 (b,d,f).

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20 (a) (b) Figure 6. Overlay of measured 6MV photon be am profile (solid line) and Monte Carlo (circle) for 10x10 and 40x40 cm2 fields at 100 cm SSD and at depths dmax (a, b), 5 cm (c, d) and 10 cm (e, f) in water, with simulated electron beam energy incident on the target was 5.70 MeV.

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21 (c) (d) Figure 6. (Continued)

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22 (e) (f) Figure 6. (Continued)

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23 (a) (b) Figure 7. Overlay of measured 6MV photon be am profile (solid line) and Monte Carlo (circle) for 10x10 and 40x40 cm2 fields at 100 cm SSD and at depths dmax (a, b), 5 cm (c, d) and 10 cm (e, f) in water, with simulated electron beam energy incident on the target was 6.0 MeV.

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24 (c) (d) Figure 7. (Continued)

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25 (e) (f) Figure 7. (Continued)

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26 (a) (b) Figure 8. Overlay of measured 6MV photon be am profile (solid line) and Monte Carlo (circle) for 10x10 and 40x40 cm2 fields at 100 cm SSD and at depths dmax (a, b), 5 cm (c, d) and 10 cm (e, f) in water, with simulated electron beam energy incident on the target was 6.30 MeV.

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27 (c) (d) Figure 8. (Continued)

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28 (e) (f) Figure 8. (Continued)

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29 2.4.3 6MV Spectrum and Fluence The photon spectrum calculated here for the region 0 r 3 cm inside a 10x10 cm2 field at 100 cm SSD is shown in Figure 9. Figure 10 shows the energy fluence distribution for the 5.7, 6.0 and 6.3 MeV beam calculated at the surface of th e phantom at 100 cm SSD, 10x10 cm2 field of mixed photons and charged particles. The 5.7 MeV, 6.0 MeV and 6.3 MeV electron beam striking the bremsstrahlung target resulted in mean energy of 1.45 MeV, 1.55 MeV and 1.58 MeV beam at the phantom surface respectively. Figure 11 (a) shows that the mean energy distribution at the phantom surface for 10x10 cm2 field is relatively flat compared to that for 40x40 cm2 field. Figure 11 (b) shows the mean energy distribution across 40x40 cm2 field decreased with off axis distance toward the field edge. The mean energy at the fiel d edge was 0.25 MeV lower compared to that at central axis for all beams. There is more low energy scatter dose contribution near the field edge compared to centr al axis for large fields. Figure 12 (b) shows the fluence increased with 20 cm off axis distance reaching 135 % of central axis, near the field edge. These are mostly lower energy particles that were attenuated by thinner part of the flattening filter toward the field edge resulting in relatively flat energy fluen ce profile across the 40x40 cm2 field, as in Figure 13.

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30 Figure 13 shows the size of the horn of energy fluence profile across 40x40 cm2 field decreased with increasing energy of primary el ectron beam incident on the target. This is because the relative intensity of photons in creases with energy. Fluence of forward directed photons down the axis at small angles increases with energy more than it does at larger angles and away from central axis towa rd the field edge. Therefore the size of the horn decreases with increasing en ergy, as seen in Figure 13. Figure 9. Calculated photon spectrum in the form of planar fluence histogram for the region 0 r 3 cm inside a 10x10 cm2 field at 100 cm SSD.

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31 Figure 10. Calculated energy fl uence distribution for 10x10 cm2 field. The simulated electron beam energy incident on the targ et was 6.30 MeV top cu rve, 6.0 MeV middle curve and 5.7 MeV the bottom curve.

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32 (a) (b) Figure 11. Calculated mean en ergy distribution across (a) 10x10 cm2 field and (b) 40x40 cm2 field. The simulated electron beam energy incident on the target was 6.30 MeV top curve, 6.0 MeV middle curve and 5.7 MeV the bottom curve.

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33 (a) (b) Figure 12. Calculated fluence versus position for (a) 10x10 cm2 and (b) 40x40 cm2 field. The simulated electron beam energy incident on the target was 5.7 MeV top curve, 6.0 MeV middle curve and 6.3 MeV the bottom curve.

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34 Figure 13. Calculated energy fluence versus position for 40x40 cm2 field. The simulated electron beam energy incident on the targ et was 5.7 MeV top curve, 6.0 MeV middle curve and 6.3 MeV the bottom curve. 2.5 Conclusions Since the calculated percentage depth dose and 10x10 cm2 beam profiles were not as sensitive to changes in primary electron beam energy as were the large beam profiles, the match between measured and calculated 40x40 cm2 beam profiles were used to find the optimum electron beam parameters in commissioning of the BEAM. The optimum electron beam incident on the targ et had a radius of 1 mm with energy of 6.0 MeV respectively.

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35 CHAPTER 3 PAPER I: SKIN DOSE STUDY OF CHEST WALL TREATMENT WITH TOMOTHERAPY This study compares the dosimetric diffe rences between TomoTherapy chest wall irradiation and traditional linear-accelerator-based tangential-beam technique. TomoTherapy treatment plans with and wit hout bolus were compared with tangentialbeam plans. Plans were also generated for phantom studies and point doses were measured using MOSFET dosimetry to verify the adequate skin dose. Monte Carlo simulations of static beams of both tech niques were performed and dosimetry was compared. (Jpn J Radiol. 2009; 27:355-362) 3.1 Synopsis The tangential-beam technique frequently presents challenges in radiation dose homogeneity to the target. To ensure adequate dose to the skin, bolus is often used. TomoTherapy has already been shown to impr ove target conformity and homogeneity in other disease sites(16, 17). Because of the tangential delivery technique and lack of flattening filter in TomoTherapy accelerator s, we hypothesize that during chest wall irradiation using TomoTherapy, the skin dos e will be adequate without bolus. Monte

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36 Carlo simulations and measurements confir med that beams from TomoTherapy deliver higher skin dose than a standard linear acceler ator. Skin dose also increases with the incident angle of the beams. Due to the ch aracteristics of the TomoTherapy beam and delivery technique, chest wall treatment plan s from TomoTherapy showed adequate skin dose (over 75% of prescribed PTV dose) even without bolus. 3.2 Introduction Many treatment modalities and techniques are available for the post-mastectomy radiotherapy to treat the chest wall(18-20). The most commonly used technique is opposed tangential external beams to cover all the potential tumor-bearing chest wall tissues(21). A supraclavicular field may be needed to adequa tely cover the regional nodes, if they are at risk(22). Adequate treatment of the chest wa ll using the tangential-beam technique requires: 1. Homogeneous dose distributi on over the chest wall; 2. Minimal dose to lungs, the oppos ite breast, and the heart; 3. Precise matching between the inferior bor der of the supraclavicular field and the superior border of the tangential fields; 4. Adequate dose to skin and the mast ectomy scar (about 75%~90% of the prescribed dose); 5. Adequate dose to the axillary and in ternal mammary nodes (45~50Gy), when they are at risk(21, 23)..

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37 These requirements often present challenge s for the treatment planning. For example, although photon beams of 6MV and lower provide adequate skin dose without using bolus every day, there can be poor dose hom ogeneity, especially if the chest wall separation is large(21). Higher energy photon beams improve dose homogeneity for large patients, but then bolus must be added to rais e the skin dose. Use of bolus has also been associated with increased acute skin toxicity(24). Common practice for chest wall treatments is to use bolus every other day, even with 6MV photon beams. It is important to note that if the bolus and non-bolus plans ar e rotated, then the goal is not to have 100% prescribed dose at the surface, but somethi ng closer to 80% of the prescribed dose. Superficial dose ranged from 74% to 93% by phantom measurement when incident beam angle varied from 0 to 90 degrees with bolus on/off alternatively(25). Previous investigators have shown that TomoTherapy Hi-Art (TomoTherapy Inc., Madison, WI) provides an advantage in a hi gher skin dose by using skin flash beams. Other clinical researchers have demonstr ated that TomoTherapy planning usually overestimates superficial dose by up to 10% for shallow planning target volumes (PTV)(26, 27) which needs to be accounted for while evaluating th e skin dose in TomoTherapy plans. However, good agre ement (<2.5%) between calculated and measured skin dose has also been reported(28). The area at risk represents some combination of the basal layer of th e skin and the dermal layer which contain the dermal lymphatics(29), and potential cancer cells within the lymphatics. Thus, the area at risk which is targeted is not on the surface, but 1 to 5 mm

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38 below the surface. Besides the superficial dose as studied by other groups(30), the focus of this study extends to dose gradient at shallow depths in the chest wall. In this study, clinical cases of chest wall treatment plans are compared. Phantom studies were also performed to compare the skin dose differences between the treatment techniques. Measurements of skin dose were compared with treatment plans. Monte Carlo simulations were also used to confirm the skin dose differences. 3.3 Material and Methods 3.3.1 Patient Cases A total of 5 previously treated chest wall patients were selected for treatment plan comparison. Treatment plans were generated for TomoTherapy and conventional tangential-beam technique. Besides PTV radi ation dose coverage, dos e distributions for lungs and heart were also evaluated. However, the skin dose is the primary planning parameter in this comparison.

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39 3.3.2 TomoTherapy Planning Clinical cases of chest wall treatment plans and phantom plans to simulate chest wall treatment were generated and studied. A collapsed cone convolution/superposition algorithm(31) was used for all the plans. Heter ogeneity corrections were applied. The pitch value in all the plans was set at 0.287. The modulation factor was 2.7. The plan objective was at least 95% of the PTV volume to receive the prescription dose of 50 Gy. Heart dose was limited such that less than 5% of heart volume received less than 20 Gy. Lung dose constraints were: less than 25% of the ipsilateral lung volume received 15 Gy and less than 15% of th e contralateral lung received 2.5 G y. Directional blocks were used in the TomoTherapy plans to reduce the dose to the lungs and hear t. TomoTherapy plans without directional blocks were also generated for comparison. Treatment plans for a Rando Man phantom (The Phantom Laboratory, Salem, NY) were also generated for skin dose measuremen ts. A Rando phantom is constructed with materials equivalent to soft tissues and skel eton. It simulates realistic human anatomical structures. Figure 14 shows examples of the plans.

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40 Figure 14. The TomoTherapy chest wall trea tment plans on a male Rando phantom. The upper row shows the transverse, coronal and sa gittal views of the plan without a bolus, and the lower row shows the plan with a bolu s. These plans were generated for the skin dose measurement using MOSFET dosimeters. Air gaps under the bolus are noticeable on the images of the plan with a bolus. 3.3.3 Tangential-Beam Planning Tangential-beam plans of clinical cases we re generated using the XiO planning system (Version 4.34.02.1, CMS, Inc, St. Louis, MO) for an Oncor linear accelerator (Siemens Medical Solutions USA, Inc. Malvern, PA). Two tangential beams were used in the plans. Field-in-field technique(32) was used in some of th e tangential-beam plans to improve PTV dose homogeneity. Electron beam was included in some of the cases to treat the internal mammary region. A superposition algorithm(33) was employed in the dose calculation. Heterogeneity correction wa s turned on to account for lung and bone densities. The same contour set of target vol umes and critical structures used in each TomoTherapy plan was used in the corre sponding tangential-beam plan. The photon beam energy used in the tangential-beam pl ans was 6MV, and the electron beam energy

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41 was 9 or 12 MeV. Chest wall treatment pl ans were also generated on a Rando phantom for skin dose measurement. 3.3.4 Monte Carlo Simulation An EGSnrc(7) based Monte Carlo simulation packag e for clinical radiation treatment units, BEAMnrc(34), was used to simulate linear accel erators with and without a photon beam flattening filter. The absence of a flattening filter in a TomoTherapy unit is one of the major differences compared to convent ional linear accelerator s. A total of 1108 electrons with the incident energy of 6 Me V were simulated in each accelerator head. Phase space files, in which physical paramete rs of all the particle s passing through the plane of interest is stored, were scored at the end of the secondary jaws. The phase space files were then used as radiation sources in phantom dose distribution calculations using DOSXYZnrc(13), another Monte Carlo simulation computer program for simple geometry media. Another major difference between a conventiona l linear accelerator and Tomotherapy is that the source to axis dist ance (SAD), for a conventional linear accelerator is 100 cm while it is 85 cm for TomoTherapy. Therefor e a difference of source to surface distance (SSD) of 15 cm was also compar ed in Monte Carlo simulations. Dose in phantom versus radiation beam inci dent angle was studied to understand the dose effect at shallow depth of the helical de livery technique that TomoTherapy uses and

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42 angled beams in tangential-beam technique. The beam size used in the simulation was 55 cm2. The resolution of the dose grid along the central axis direction in the water phantom was 33 0.15 cm3, where 0.15 cm was along the dir ection perpendicular to the phantom surface. The rotation axis of the beam s (isocenter) was at the phantom surface. 3.3.5 Dose Measurements The mobileMOSFET TN-RD-16 wireless dose ve rification system (T homson & Nielsen Electronics Ltd, now Best Me dical Canada, Nepean, Ontari o, Canada) was used for the dose measurements. According to the specificati ons, the system has an accuracy of 2% at 200 cGy dose level at standard bias. The MOSFET readings were cross checked with ion-chamber measurements using the TomoTherapy monthly static output in refe rence geometry set up. With this setup(35), stationary dose delivery was used to de liver about 200 cGy to the depth of maximum dose, dmax, of a solid water phantom. Chambe r reading and MOSFET reading were acquired simultaneously. Point doses of the TomoTherapy plans were measured at the phantom surface and under 5 mm bolus. TomoTherapy and tangential-be am chest wall plans on the Rando phantom were delivered on a TomoTherapy unit and an Oncor linear accelerator respectively.

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43 Treatment plans for a TomoTherapy deli very quality assurance (DQA) phantom, known as cheese phantom, were also generated for this study. The cheese phantom was used because of the convenience of using a ready-pack film to measure the dose distribution including the “s kin” dose. Kodak extended do se range (EDR) films were used to measure the skin dose gradient rela tively. Figure 15 shows the measurement setup with film and MOSFET dosimeters. A Hurter and Driffield (H & D) curve was generated for this purpose. The exposed EDR film was scanned and the dose distribution image was analyzed on the TomoTherapy planning sy stem. The measured dose distribution was aligned with the planar dose di stribution from the plan using the alignment tools in the planning system. Figure 15. Film and MOSFET dose meas urement setup with TomoTherapy cheese phantom. Treatment plans were generated to simulate chest wall treatment. The treating region was along the edge of the phantom where MOSFET dosimeters were located.

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44 Figure 16. Dose gradient difference between the film measuremen t and the treatment plan. The real dose increases with depth much faster than that in the plan. Due to this difference and the coarse resolution in TomoTh erapy treatment plans, a slight difference in phantom edge definition could cause a large range of dose variation between measurements and plans. In this figure, th e difference between line a and d is about 2 mm, while the dose difference of the planne d dose-measured dose ranges from -0.1 to +0.4 Gy. 3.4 Results 3.4.1 Film Dosimetry The relative dose distributions measured us ing EDR film on the cheese phantom for chest wall treatment plans showed st eeper dose gradient than th at in TomoTherapy plans. Figure 16 is an example of the comparisons of the dose gradient s between the planned and measured dose distributions. In this exam ple, the shallow dose increased from 1.5 to 2.0 Gy in 2 mm depth. Due to the coarse spatial resolution in TomoTherapy planning system, one pixel misalignment between the film and the phantom image could introduce 2 mm spatial shift. If this sp atial shift is along the high grad ient direction, it corresponds

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45 to a 0.5 Gy difference in superficial dose. Th e dose gradient differe nce between the film measurements and TomoTherapy plans could be the explanation to the wide variation of superficial dose differences reported by different groups(26-28) (see Figure 16). Also because of this difference, TomoTherapy planning is likely to overestimate the superficial dose. An important conclusion of the film dosimetry study is that the TomoTherapy planning may overestimate the s uperficial dose while it under estimates the dose gradient in shallow regions in chest wall treatment pl ans. In the example shown in Figure 16, the dose gradient of the measured dose is two times that of the planned dose in the high gradient region. The sharp dose gradient in chest wall trea tment may cause higher skin dose (not superficial dose) th an what the plan shows.

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46 Figure 17. (A) Monte Carl o calculated TomoTherapy pe rcentage depth dose (PDD) versus measured PDD. The SSD is 85 cm; (B) Monte Carlo calculated shallow depth dose distributions of a TomoTherapy machin e and a conventional linear accelerator versus SSD and incident angle. The statistical uncertainty of the Monte Carlo calculated PDD is within 1% for all data points (see the error bars).

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47 Table 1. Comparison of the measured s uperficial dose between TomoTherapy and tangential-beam techniques on a Rando phantom using MOSFET. Technique TomoTherap y Tangential-beam Location Surface 5mm bolus Alternative Surface 5mm bolus Alternative Dose (cGy) 152.46.1 205.69.7 179. 05.7 134.08.0 188.55.5 161.34.9 % to 2 Gy 76.23.0 102.84.8 89. 52.9 67.04.0 94.32.8 80.62.4 3.4.2 MOSFET Dose Measurement The calibration of MOSFET against ion cham ber was carried out using a TomoTherapy unit in stationary mode. The MOSFET readings differed with ion chamber readings by 1.51% 0.44%, within the manu facturer’s specification. Table 1 lists the measured point doses at phantom su rface and under 5 mm bolus. The average dose of alternative bolus on/off was calculated using the measured doses. The measured superficial dose of the TomoTher apy plan (76.2% 3.0% of the prescribed PTV dose) is lower than the alternative bolus on/off dose of the tangential-beam technique (80.6% 2.4%) but comparable, wh ile the alternative on/off dose from the TomoTherapy plan (89.5% 2.9 %) is also within the adequate dose range. Considering the skin dose is actually not the superficia l dose but at about 1 mm depth, and the sharp dose gradient in the shallow re gions, this dose could be too high and likely to cause skin reactions.

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48 3.4.3 Monte Carlo Study Figure 17A shows the agreement of the meas ured and calculated PDD curves for the TomoTherapy machine. Figure 17B demonstrat es the dose differences at shallow depth versus SSD and accelerator. The effect of the 15 cm difference in SAD between TomoTherapy and conventional accelerator can be seen in this figure. About 2% difference in dose is introduced by the SAD di fference, with a highe r superficial dose for a shorter SAD (the superficial dose differen ce between curves 1 and 4 in Figure 17B). The absence of the flattening f ilter increases the superficial dose by about 7% at the same SSD (Curves 3 and 4). The combination of th e absence of a flattening filter and shorter SSD results in about an 8% higher superficia l dose for TomoTherapy (Curves 1 and 3). Tangential beams increase the superficia l dose for both TomoTherapy beams and conventional accelerator beams. The Monte Carl o simulation shows that the magnitude of the increase is about the same for both Tomo Therapy beams and conventional accelerator beams (Curve 2 versus Curve 1 for TomoTher apy beams and Curve 5 versus Curve 3 for conventional accelerator beams in Figure 17B). A 17% superficial dose increase can be observed in Figure 17B when the incident angle is changed from 0 to 60 degrees. The shallower dmax in TomoTherapy beams with large in cident angles is the major reason that the TomoTherapy chest wall treatment plans usually have higher skin dose than conventional tangential-beam plans.

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49 3.4.4 Plan Analysis and Comparison Figure 18 shows the skin dose comparison of a TomoTherapy plan without bolus to tangential-beam plans of with and without bolus. The same patient and same location was chosen for all the depth dose profiles of th e TomoTherapy and tangential-beam plans. The superficial dose of the TomoTherapy plan was much higher than the average of the two tangential-beam plans. While Figure 17 shows the comparison for only one location, Table 2 lists more sta tistical data of the comparison, wh ich shows even higher skin dose without bolus and less dose variation in the TomoTherapy plans compared to the conventional tangential-beam pl ans. Comparing the superficial doses listed in Table 2 with the ones in Table 1 which are the measur ement data, an overestim ation of superficial dose in TomoTherapy plans of about 12% can be concluded, while for the tangential plans, the overestimation is about 2%. Taking into account the overestimation of superficial dose by TomoTherapy, in the ex ample shown in Figure 18, even without bolus, the corrected TomoTher apy plan (about 80% superfic ial dose) still provides adequate skin coverage, which agrees with th e measurement data in Table 1 within 4%.

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50 Figure 18. Skin dose comparison of TomoTher apy with tangential-beam technique. All dose profiles were obtained from treatment plans of the same left sided chest wall patient at the same location. The superficial dose of the TomoTherapy plan (without bolus) is between the ones of tangential pl ans with and without bolus. Table 2. Normalized dose at the surface, 2 and 5 mm depth is compared for chest wall treatment plans using the tangentia l-beam technique and TomoTherapy. Dose at the surface and depth are normalized to the prescribed dose. A total of 20 points in treatment plans were chosen for each depth. The value s hown in this table for each plan at each depth is the average and one standard devi ation. Even without bolus, TomoTherapy plans usually have high superficial dose and dose at shallow depth (skin dose). The standard deviation in TomoTherapy plan s is usually smaller than th at in conventional tangentialbeam plans, indicating more conformal dose distributions in TomoTherapy plans. The overestimation of superficial dose in the plans is not corrected in this table. Plan % superficial dose % dose @2mm % dose @5mm Tangential, no bolus 69 6 85 5 97 3 Tangential, bolus 83 7 87 4 95 2 TomoTherapy no bolus 88 3 92 4 94 4 TomoTherapy bolus 102 3 104 2 101 2

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51 3.4.4.1 Discussion A possible problem associated with bolus is that there may exist air gaps between the bolus and chest wall (Figure 14) which could further vary in daily treatments. This variation would introduce uncer tainty in delivered dose. Especially for TomoTherapy, the variation of daily bolus location could intr oduce dose uncertainty due to its helical delivery technique. However, bolus has been suggested as desirabl e for TomoTherapy to reduce the effect of daily setup error and potential underdosing of the surface(36), and to correct for shallow depth dose overestima tion in TomoTherapy treatment planning algorithm(26). Due to the angular dependence of MOSFET dosimeters (3.0-3.5%), their measurement uncertainties (2-3%) and other possible set up errors, the estimated accuracy of MOSFET dosimetry is about 6%(37). The estimated 12% overestimation of TomoTherapy treatment planning system may include the MOSFET measurement uncertainty. Based on the measurement data, TomoTherapy without bolus delivers adequate skin dose but it is lower than what the alternative bolus on/off plan of tangential-beam technique delivers (76.2% 3.0% of pr escribed PTV dose versus 80.6 % 2.4% to skin surface). TomoTherapy alternative plan also delivers adequate skin dose (89.5% 2.9% to skin surface) but at the high limit. A scheme of tw o days without bolus and one day with bolus in TomoTherapy gives a moderate superficial dos e (85.1% 2.6%).

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52 3.5 Conclusion Compared to a tangential-field technique, TomoTherapy shows a higher superficial dose in chest wall treatment wit hout using bolus in both treat ment plans and measurements. The reasons of the higher superficial dose are 1) slightly lower mean energy than a conventional linear accelerator and therefore a shallower dmax, 2) shallow delivery angles used in the treatment delivery, and 3), th e smaller SAD of the TomoTherapy unit. Therefore it is a reasonable clinical practice to treat ch est wall on TomoTherapy without using bolus, or using a modified bolus on/o ff scheme (two days off, one day on).

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53 CHAPTER 4 PAPER II: COMPENSA TOR-BASED INTENSITY-MODULATED RADIATION THERAPY FOR MALIG NANT PLEURAL MESOTHELIOMA POST-EXTRAPLEURAL PNEUMONECTOMY This chapter investigates the potential of compensato r-based intensity-modulated radiation therapy (CB-IMRT) as an alternative to multileaf collimator (MLC)–based intensity-modulated radiation therapy (IMRT) to treat malignant pleural mesothelioma (MPM) post-extrapleural pneumonectomy. Th is study points out the challenges in planning and delivery of large fields on a sp ecific linear accelerator with MLC. The study focuses on producing dosimetrically acceptable and deliverable IMRT plans for treating large modulated fields with solid modulator and step and shoot MLC. These plans are also calculated on a phantom for the quality assurance test of absolute point dose and relative dose distributions. (J Appl Clin Med Phys. 2008 Oct. 29; 9(4): 98-109) 4.1 Synopsis Treatment plans for four right-sided and one left-sided MPM post-surgery cases were generated using a commercial treatment planning system, XIO/CMS (Computerized Medical Systems, St. Louis, MO). We used a 7-gantry-angle arrangement with 6MV beams to generate these plans. Th e maximum required field size was 30x40 cm2. We

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54 evaluated IMRT plans with brass compensators ( Decimal, Sanford, FL) by examining isodose distributions, dose–volume histograms, metrics to quantify conformal plan quality, and homogeneity. Quality assurance was performed for one of the compensator plans. Conformal dose distributions were achieved wi th CB-IMRT for all 5 cases, the average planning target volume (PTV) coverage be ing 95.1% of the PTV volume receiving the full prescription dose. The average lung V20 (volume of lung rece iving 20 Gy) was 1.8%, the mean lung dose was 6.7 Gy, and the average contralateral kidney V15 was 0.6%. The average liver dose V30 was 34.0% for the right-sided cas es and 10% for the left-sided case. The average number of monitor units (MUs) per fraction was 980 MU for the 45Gy prescriptions (mean: 50 Gy) and 1083 MU fo r the 50-Gy prescriptions (mean: 54 Gy). Post-surgery, CB-IMRT for MPM is a feasib le IMRT technique tr eated with a single isocenter. Compensator plans achieved dose obj ectives and were safely delivered on a Siemens Oncor machine (Siemens Medical Solutions, Malvern, PA). These plans showed acceptably conformal dose distributions confirmed by multiple measurement techniques. Not all linear accelerators can deliver large-fi eld MLC-based IMRT, but most can deliver a maximum conformal field of 40x40 cm2. It is possible and reasonable to deliver IMRT with compensators for fields this size with most conventional linear accelerators.

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55 The future work will address the skin dose of compensator-based IMRT treatments. Key words: malignant pleural mesothelio ma, compensator-based IMRT, SMLC IMRT, plan conformality, quality assurance 4.2 Introduction Treating malignant pleural mesothelioma (MPM ) post-surgery requires very large fields. This paper addresses intensity-modulated radiation therapy (IMRT) plans with solid modulators for the large fields required to tr eat MPM post-surgery, given that these plans first closely achieved the pres cription dose objectives, passed the quality assurance (QA), and were safely delivered. Malignant pleural mesothelioma is a fatal aggressive cancer of the pleura and a large complex target volume. Reports show that the incidence of the disease is increasing globally, with 2000 new cases annually in the United States(38). Increased incidence of mesothelioma is strongly associated with e xposure to asbestos, which is most commonly used in Western industrial societies; more men than women are affected(39). In 2003, the Surveillance, Epidemiology, and End Results Program of the U.S. National Cancer Institute projected the total number of MPM cas es in American men to be approximately 71,000 by the year 2054(40). Because of the predicted numbers of new cases, the National Cancer Institute is sponsoring clinical trials designed to seek new treatment modalities.

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56 Traditionally, radiation therapy treatment techniques used exte rnal beam radiation with a combination of photon and electron beams(41, 42) and intraoperative brachytherapy with post-operative mixed photon irradiation(43). Normal tissue was spared using photon and electron blocks for external beam treatme nts. Various dose regimens have been prescribed for palliation and local control of this disease, ranging from 30 Gy(44) to a median dose of 36 Gy(45) (palliation) and 54 Gy (45 – 54 Gy, local control)(46) administered to the hemithorax. The latter treatment showed improve d local control with acceptable toxicity. This finding seems to demons trate that a sufficient dose was achieved for palliation and local control of the disease with the conventional techniques. Published literature lacks metrics, including dos e–volume histograms (DVHs), which have increasingly become a crucia l part of plan review, a nd comparisons complementing isodose distribution in transverse and ort hogonal planes. Radiation oncologists often use information from computed tomography (CT), magnetic resonance, and positronemission tomography imaging to accurately delin eate the target and organs-at-risk (OAR) volumes so as to prescribe and quantify the dose to these sensitive overlapping structures. The use of IMRT allowed for further dose escalation to large target volumes while maintaining tolerance doses to abutting radiation-sensitive structures(47). Post-operative IMRT for MPM has shown the most promisi ng early local control of this disease(48-50). Current techniques often couple IMRT from a specific treatment pl anning system with specific beam delivery and verify systems. Stevens et al.(67) found that Corvus, Pinnacle, and Eclipse treatment planning systems were all capable of generating acceptable IMRT plans for MPM after extrapleural pneumonectomy (EPP). The authors compared treatment planning systems and found that th e early plans with Corvus had the largest

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57 number of monitor units (2786 MUs) and se gments (1050 segments), and that a newer version of Eclipse had the least number of MUs (1813 MUs) and segments (173 segments). Delivery of large IMRT fields with a multileaf collimator (MLC) is limited by MLC designs(51). For example, the Siemens Oncor machine (Siemens Medical Solutions, Malvern, PA) with 82-leaf Optifocus MLC sy stem allows for a maximum IMRT field size of 22x40 cm2. The MLC carriage-over-travel distan ce past the central axis is limited to 10 cm. Even though a field size of 24x40 cm2 can be accommodated, given that the smallest segment size can be set to 2x2 cm2, larger IMRT field widths are required to treat MPM. To overcome the MLC field size limitations, treatments with multiple isocenters have been propos ed by other investigators(19). We investigated a compensatorbased IMRT (CB-IMRT) technique with a si ngle isocenter to treat MPM post-surgery. It is essential that the modulator (MLC or solid brass compen sator) reproduce the intended fluence map. For three of four ri ght-sided cases, a numb er of IMRT fields required a minimum field width of 26 cm. These cases were good candidates for CBIMRT delivery (which has no IMRT fiel d size limit, and for which a maximum conformal field size of up to 40x40 cm2 is possible) on the Siemens Oncor machine. Intensity-modulated radiation therapy with co mpensators has been successfully used for more than a decade(52,53). The CB-IMRT technique offers continuous intensity modulation. Compensators deliv er the intensity-modulated dos e in static form to all

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58 points within a field relatively instantaneously where the beam-on time depends on the machine dose rate. To investigate the potential of very large field CB-IMRT for MPM, the goal was to create plans that closely achieved the prescripti on dose objectives for MPM post-surgery and that produced manageable modulators that were delivered on our Siemens Oncor treatment machine. 4.3 Materials and Methods All data sets acquired for this test st udy came from patients who underwent surgery before simulation. 4.3.1 Surgery The EPP procedure involves removal of the ipsilateral lung (rem ove motion) and hemidiaphragm resection, with subsequent reconstr uction using polytetraf luoroethylene fabric. A mediastinal lymph nodes dissection is also performed, as is a chest wall resection and reconstruction. To assist the radiation oncologi st with the contouring, the surgeons place clips to identify the entire outline of the resected hemi-diaphragm and the resected margins. These areas are otherwise difficult to identify post-operatively. The simulation for treatment planning occurs 6 – 8 weeks post-surgery.

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59 4.3.2 Simulation Surgical scars were typically wired and th en covered with bolus (7 cm wide, 0.50 cm thick) extended 4 cm proximally and distally over the scar. Patients were immobilized supine on the CT couch using a wing board (M ed-Tec, Orange City, IA) in combination with a vacuum bag and the T-bar system indexed to the couch top. The T-bar helps support the arms up and out of the radiati on field. Radio-opaque ball bearing markers were taped to the anterior and lateral sides of the patient for treatment planning and as a setup reference. Simulation CT slice thicknesses were typically 5 mm (no larger). At least 100 transverse slices were acquired and transferred to the treatment planning system. 4.3.3 Contours The use of IMRT required contouring of th e clinical target volumes (CTVs), which included the tumor bed (post EPP) and the regi ons at risk for seeding of disease. The CTV extended from T1–L3 (from apex of thorax to inferior pole of kidney). The contours for the contralateral lung, kidneys, heart, li ver, esophagus, small intestine, spinal cord, and skin were drawn on every slice (a timeconsuming process). The three dimensional auto margin functions were used to crea te a PTV0.5 cord (CTV + 0.5 cm) and cord avoidance (cord + 0.5 cm) structure. Avoi dance and boost structures were drawn manually. To account for uncertainties in cont ouring the CTV, superior and inferior

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60 margins were set 1 cm above and 1 cm below th e most superior and mo st inferior surgical clips. The anterior, posteri or, and lateral margins were defined by adding 0.75 cm to violated spaces clipped at the skin. 4.3.4 Treatment Planning Coplanar IMRT beams (6MV) aimed at the ce nter of the planning target volume (PTV) were designed. The first gantry angle was 180 degrees, and the remaining 6 angles were at about 30to 45-degree intervals, excludi ng the anterior–posterior field. These were selected using a beam’s eye view tool to minimize entrance and limit exit doses to contralateral lung. Gantry angles were adjust ed as much as possibl e while obtaining the desired dose distributions. We noticed that li ver position varied in the superior–inferior direction with respect to CTV position for th ese cases. Planning right-sided cases with sufficient CTV coverage, given the extent of liver in the radiation field, was more challenging because of the competing dose constraints of these structures. 4.3.5 IMRT Plans Seven fields (gantry angles), A – G, were selected to conf orm to PTV using MLC. Fields A – D were split to keep the compensator weights manageable, resulting in additional fields A1 – D1. Fields A and A1 were split in the inferior–superior direction with at least a 2 cm overlap margin. The PTV contour seen by each field was also edited and MLC conformed to the new split contour shapes. Th e same procedure was repeated for fields

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61 that needed to be split. The fi elds were split at varying distan ces from central axis to help reduce possible high dose at the junction area. 4.3.6 IMRT Prescription Page Target and OARs were set up in the IMRT pr escription page according to the prescription guidelines shown in Table 3. Deviation fr om goal doses for the minimum and the maximum PTV coverage was given the highest penalties. Liver, kidneys, contralateral lung, and heart were given higher overlap prio rities than were the other OARs. Maximum dose to the liver was set to the maximum PTV dose with relaxed penalty, but certain percentages of liver and contra lateral lung volumes were restri cted to very low doses with very high penalties. The point was to achie ve the suggested prescr iption dose ob jectives to these structures.

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62 Table 3. Dose–volume guidelines for the target and organs at risk (OARs)a. Target or OAR Dose–volume guideline Clinical target volume V100 > 98% Planning target volume V100 > 95% Contralateral lung V20 < 4% Mean lung dose 6–8 Gy or ALARA Spinal cord Less than 10% > 45 Gy 0.0% > 50 Gy Heart V45 < 50% Liver V30 < 30% or ALARA Right kidney V15 < 20% Left kidney V15 < 20% Esophagus V55 < 30% a, The planning target volume is the clinical target volume plus 0.5 cm. The V100 is the volume receiving 100% of the prescribed dose. Contralateral lung V20 is the volume of lung receiving 20 Gy. Lung mean lung dose and liver V30 are kept as low as reasonably achievable (ALARA). 4.3.7 Compensator Plans 4.3.8 Treatment Planning Strategy Our CB-IMRT plan strategy process started with an arrangement of 5 non-coplanar fields for the left-sided case and arrangements of 7 coplanar fields for the right-sided cases, which resulted in acceptable dose distributions but very heavy modulators (weight up to 15.9 kg). An 11.3-kg compensator from the 7-fi eld arrangement is shown in Figure 19. These large fields were then sp lit, which resulted in 7-gantry 14-coplanar fields with field junction overlap matched at the central axis using modulators 7.62 cm thick that yielded acceptable dose distributions with 125% hot spots in the junction area. The average

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63 compensator weight was 8.2 kg (range: 6.3 – 9.1 kg). We did not achieve the plan objectives if all beams were modulated with the brass modulators (5.08 cm maximum thickness), even with the MLC blocking. The outcome was a 7-gantry 11-coplanar field arrangement with modulators (5.08 cm and 7.62 cm ma ximum thickness) and field junction overlap at varying di stance from the central axis. The median compensator weight was 7.5 kg (5.2 – 10.0 kg). All CB-IMRT plans were generated using an MLC blocking that conformed the beam to the PTV plus a 0.5 cm block margin. We found it desirable to plan th e compensator fields with MLC blocking advantages. In addition, individual MLC leaves can be edited to further enhance the distribution even after th e compensators are generated. The effective attenuation coefficient (EAC) values were assi gned for all fields. We used compensators (7.62 cm maximum thickness) with an inte nsity modulation range of 8% – 100% for fields that contained significan t parts of the contralateral lung or liver. These large fields would produce heavy modulators, and thus we re divided. We used compensators (5.08 cm maximum thickness) with an intensity modulation range of 20% – 100% for fields that contained no significant parts of lung or liver. The only lung had to be spared. We tried to keep the 10-Gy isodose line outside the contralateral lung and kidney volumes to maintain prescription guidelines to these structures. All IMRT plans were calculated using superposition with hetero geneity corrections applied. Dose distributions and DVHs were reviewed by the radiation oncol ogist. Each field was set up in the IMPAC Record and Verify system (IMPAC Medical Systems, Sunnyvale,

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64 CA) with its unique accessory tray code (S2N01–S2N18). Tw o coded trays are shown in Figure 19. Compensator thickness file s were electronically sent to Decimal for fabrication; the modulators we re returned within 24 hours. Figure 19. Two modulators from Decimal mounted on the Siemens coded trays. One of the large compensators (11.3 kg) from the in itial 7-field plan th at did not require extensive blocking within the fi eld is shown next to one of the 7-field IMRT modulators for prostate (1.8 kg). 4.3.9 Compensator Thickness File The brass thickness t ( i j ) can be calculated as a simple exponential attenuation equation for an array of values each representing th e filter thickness for each ray line at ( i j ) as in equation 2: effj i trans j i t, ln (2)

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65 where Trans( i j ) is the transmission for each ray line throughout the compensator and eff is the EAC for solid brass compensator under broad-beam geometry. du Plessis et al.(54) showed that the absorbed dose varies exponentia lly as a function of ab sorber thickness on the beam axis at any depth in water for any material. These authors showed that EAC for brass can vary by as much as 13% over a depth range of 4 – 39 cm. The measurement showed that EAC values decreased with in creasing field size, depth, and thickness as beam hardening and more scatter for larger field sizes contributed to dose at the given depth. Beam divergence and beam hardening is ta ken into account by the treatment planning system dose computation. Mean energy of th e beam increases after the beam passes through the modulator. Jiang and Ayyangar(55) showed that beam hardening resulted in greater change (sparing) in surface dose than in dose at depth with respect to maximum dose. Authors showed that for a 6MV 10x10 cm2 beam, a 10% maximum dose reduction occurred in surface dose and a 3% dose increas e occurred in percent depth dose at 10 cm depth for Cerrobend slab 5 cm thick ( p = 9.76 g/cm3). 4.4 Safety Considerations We considered limiting the maximum compensator weight to a level that therapists were comfortable with in safely handling the devi ce. We had the therapists try the 10 kg and 15.9 kg compensators. The therapists who coul d easily handle the 45-degree solid wedge (which is 6.1 kg) were able to insert the 10 kg modulators in to the wedge slot with ease;

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66 the 15.9 kg modulators posed more of chal lenge for most of the therapists. The manufacturer’s recommended weight limit for the Siemens Oncor block tray accessory is 15.9 kg; however, we do not have the weight limit information for the wedge tray slot accessory. We tried to exercise safety in handling the relatively heavy modulators. Modulators were loaded with the gantry set at 90 degree s or 270 degrees. The delivery was such that no loaded compensator field crossed over the patient at any time, and for patient safety, plans were designed w ithout the anterior–posterior field. 4.4.1 Plan Evaluation We used two-dimensional isodose distributions (axial, sagittal, a nd coronal planes) to visually inspect the target coverage. Quantitative techniques such as DVH analysis and other indices were used to ev aluate the plans. Table 4 s hows the fraction of PTV volume receiving 100% of the prescr ibed dose in grays (PTV V100); high dose in grays to 5% of PTV volume (PTV D05); conformation number (CN), an index proposed by van’t Riet et al.(56) which takes into account the quality of coverage of the target and the volume of healthy tissue receiving at l east the prescribed dose; and the homogeneity index (HI) as the ratio between maximum dose and prescrip tion dose within the target. In addition, contralateral lung V20, V5, and mean lung dose (MLD); V30 for liver; V45 and V50 for heart; V15 for contralateral kidney; and maximum cord dose and dose to 10% of cord ( D10) were used to score OAR protection. Treatm ent efficacy with compensators in terms

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67 of MUs per field, total MUs, and beam-on time per daily fraction and per total treatment time are reported. 4.4.2 Quality Assurance We used multiple measurement techniques to perform IMRT QA. Dose distributions for one of the compensator plans were recalcula ted for three QA phantoms. This repetition provided the reference for comparison with ab solute point dosimetry measurement using a calibrated ion chamber, single coronal field using absolute dose distributions measurement with the Map Check diode a rray device (Sun Nuclear Corporation, Melbourne, FL), and composite dos e distributions at 4 transverse film planes irradiated simultaneously with true gantry angles inci dent on the phantom. We registered extended dose range (EDR2) films to the plan and anal yzed them using the RIT 113 film dosimetry system (RIT Inc., Denver, CO). 4.5 Results Figure 14 (a–d) shows isodose distributions fo r right-sided and left-sided cases. Notice that the 10-Gy isodose line is kept outside the contralate ral lung. Figure 14 (a,b) shows dose distributions in the corona l and sagittal planes, and dose profile at various distances from the central axis in those planes. Figure 14 (a) shows the corona l dose distributions for the right-sided case. The flat dose pr ofiles show that dose varies 2% across the coronal plane and 6% along the plane. Figure 14 (b) shows the dose distributions for the

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68 right-sided case in the sagitt al plane. The profile througho ut the field junction shows 118% hot spots. Figure 14 (c) shows the 50Gy prescription dose distributions for the right-sided case. Figure 14 (d) shows the dose distributions fo r the left-sided case. Figure 15 (a,b) shows DVHs for one of the right -sided and the left-sided MPM case. (a)(b) (c)(d) Figure 20. Dose distributions and profiles in th e coronal and sagittal planes. (a) Sagittal profile of a right-sided case s hows the profiles across the coronal plane at isocenter, 2 cm from isocenter, and 6 cm inferior and 12 cm superior to isocen ter. The profile 6 cm inferior to isocenter shows the degree of live r-sparing in this plane. (b) Coronal profile of a right-sided case shows the profiles at isoc enter, 2 cm from isocenter, and 5 cm posterior and 8 cm anterior to isocenter plane. (d) Dose distribution for the left-sided case is shown.

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69 All plans conformed to 99.2% of CTV volume and 95.5% of PTV volume achieving the prescription dose. Lung V20 was less than 2%, and MLD ranged between 5 Gy and 8 Gy for all plans. All lung MLDs and V5s were below the range at which pneumonitis was no longer reported to have been observed by other investigators(57). The average value for liver V30 was 34%. To compare IMRT treatment parameters wi th segmented MLC (SMLC) and compensator delivery on the Siemens machine, we used th e same field configur ation in planning 2 cases with SMLC IMRT. Large optimized flue nce had to be segmented with a minimum MLC segment size greater than 2 cm2 (because of IMRT field size limitations) to produce deliverable plans. More desira ble and better dose delivery resolution was not possible in this case. In Table 4, note that ( TVRI/ VRI) is worst for plan 5 with MLC (5LM). As compared with compensator plan 5LC, this plan shows more non-target tissue receiving the prescription dose.

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70 (a) (b) Figure 21. Dose–volume histograms for the planni ng target volume (PTV), clinical target volume (CTV), and the liver, lung, kidneys, and spinal cord for (a) a right-sided case, and (b) the left-sided case. One possible explanation may be that plan 5L M needed to have a smaller segment size, and thus higher dose sculpting power, to block portions that needed to receive less dose. This plan also had higher HI than did th e compensator plan. Table 4 also shows MLC parameters such as total number of segments and MUs, and total treatment time. Total number of MUs was 1993 MUs with 244 segmen ts for plan 1RM (Table 4). Treatment time per beam can be seen to be significantl y shorter for compensators. The total number

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71 of MUs was doubled for SMLC. Total treat ment time was slightly shorter for compensator delivery, and yet comparable w ith automated SMLC delivery as shown in Table 3. Table 3 shows the number of MU s for the 5 compensator and 2 MLC plans. Our plans for 4 right-sided and 1 left-sided case result ed in an average of 980 MU (range: 882 – 1040 MU) per daily fraction. The average daily de livery time was 33 minutes, which included entering the room to check the isocenter and to replace the compensators for all fields. Table 4. Plan valuesa. Plan PTV PTV (TVRI/TV) (TVRI/VRI) CN HI ID V100 (%) D05 (Gy) 1RC 95 52 0.95 0.96 0.92 1.3 1RM 95 55 0.95 0.96 0.91 1.4 2RC 95 54 0.95 0.82 0.78 1.5 3RC 95 54 0.95 0.90 0.86 1.2 4RC 95 56 0.95 0.93 0.88 1.2 5LC 97 52 0.97 0.85 0.83 1.2 5LM 98 55 0.98 0.81 0.80 1.5 a.Plan 1RC (plan 1, right-sided case, with compensators) shows that 95% of the planning target volume (PTV) received 100% ( V100) of the prescribed dose of 45 Gy The high dose to 5% of the PTV volume is 52 Gy for the compensator plan and 55 Gy for the multileaf collimator (MLC) plan 1RM (plan1, right-sided case, with MLC). PTV = planning target volume; TV = target volume; RI = reference isodose line; CN = conformation number; HI = homogeneity index; V100 = volume receiving 100% of the prescribed dose; D05 = high dose to 5% of the volume; plan ID key: plan number (1 – 5), rightor left-sided (R, L), compensator or multileaf collimator (C, M).

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72 Table 5 Plan delivery valuesa. Plan Average Total Segments Average Total ID MUs MUs beam-on time treatment per field per field time (s) (min) 1RC 89 976 — 18 33 1RCb 98 1083 — 23 34 1RM 181 1993 244 198 36 2RC 87 962 — 18 33 3RC 94 1040 — 19 35 4RC 97 1039 — 19 35 Avg. RC 92 1004 — 18.5 34 5LC 80 882 — 16 30 5LM 164 1801 193 126 35 a.The average beam-on time per field for compensa tor plans 1RC – 4RC and 5LC was calculated based on 300 cGy/MU at the central axis. The number of segments are shown for the multileaf collimator plans 1RM and 5LM. Total treatment time for compensator delivery includes entering the treatment room to replace the compensator for each field. Plan ID key: plan number (1 – 5), rightor left-sided (R, L), compensator or multileaf collimator (C, M). b.For 50 Gy prescription dose. 4.5.1 QA Results The measured absolute dose in water and in cube phantom for the composite plan agreed within 3% of the calculated dose. Figure 16 (a) shows one of the i ndividual fluence maps from Map Check. At least 95% of the measur ed and calculated isodos e distributions were found to be in agreement within 3% and 3 mm distance to agreement for all individual compensator fields. The large EDR2 film dosimetry showed 117% hot spots in the junction area, consistent with the predicte d value. Figure 16 (b) shows the QA results

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73 using the cube phantom. The measured orthogon al profiles were extracted, which showed good agreement with the calculated profiles. (a) (b) Figure 22. The calculated and measured isodos e distributions in th e coronal plane for one of the compensator fields is shown in the top right and top left quadrants. At the bottom left, the overlaid absolute dose distri butions are seen. Over laid oblique profiles are shown in the bottom right quadrant. DTA = distance to agreement. (b) The cube phantom is shown at the lower left. The RIT film dosimetry system (RIT Inc., Denver, CO) analysis window shows good agreement be tween the measured and calculated dose distribution.

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74 4.6 Discussion Although the words “compensator” and “solid modulator” were used interchangeably throughout this chapter, the intention was to re fer to the same device that modifies the intensity of the beam The goal of the present study was to devise a CB -IMRT technique for MPM post-surgery that met the prescription objectives and could be safely delivered. The results reported here are based on the reference dose of 45 Gy. We also tested the technique for a higher prescription dose. We found that the results also apply to the prescription dose of 50 Gy. We re-optimized plan 1RC (plan1, right-sided case, with compensators) for a higher prescription dose of 50 Gy. Figure 14 (c) shows the dose distribution. We achieved the prescription ob jectives with at least 95% of the PTV volume receiving 50 Gy, with a mean dose of 54 Gy. Liver V30 was less than 36%, contralateral lung MLD and V20 were 5.7 Gy and 1% respectively. No portion of cord received 50 Gy. The total numbe r of MUs for the 50-Gy plan for the right-sided case was 1083 MU. We used conformity number as one of the metrics to quantify conformal plan quality. The conformity number is defined in equa tion 3. We calculated the DVHs for all tissue and non-tissue (unspecified tissue not contour ed). The target volu me covered by the reference isodose line, TVRI, was calculated. The first term in equation 3 takes account only of quality of target coverage. This value for right-sided plans was 0.95 for compensator plans (as in Table 4). The s econd term describes the ratio of the PTV volume that received the prescription dose to the volume of the reference isodose line.

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75 All non-target tissue and non-tissue volume co vered by the reference isodose line was summed. The average value for the 4 right-sided cases was 0.90 for compensator plans and 7.2% of non-target tissu e for right-sided cases. Fo r left-sided case with compensators, 12.3% received the prescription dose. If the CN value is 1, the target conformity is 100% and the dose must fall ra pidly outside the large PTV. As indicated earlier, part of the liver a nd kidney volumes fell inside th e PTV because of overlap. In fact, about 10% of liver volum e received the reference dose. ) ( ) (RI RI RIV TV TV TV CN (3) 4.7 Conclusions We found that CB-IMRT with 7 gantry angl es produced dosimetrically acceptable plans for a single isocenter, without the need to match electron fields; however, it produced heavy modulators. The same gantry angles with 11 coplanar 6MV IMRT fields produced acceptable conformal plans and closely achie ved the prescription dose objectives. The resulting modulators with an eq uivalent field size of 26 cm2 were easier to manage. Total treatment time for manual CB-IMRT deliver y was comparable with automated SMLC delivery. For MPM, CB-IMRT showed accep tably conformal dose distributions confirmed by multiple measurement techniques. Not all linear accelerators can deliver large-field SMLC-based IMRT with a single isocenter, but most can deliver a maximum conformal field size up to 40x40 cm2. It is possible and reasona ble to deliver IMRT with compensators for fields this size with most conventional linear accelerators. IMRT with

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76 solid modulators adds an additional option to existing linear accelerators (LINACs) to treat large target volumes, as was the case for MPM post EPP.

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77 CHAPTER 5 PAPER III: 6MV BUIL DUP DOSE FOR COMPENSATOR-BASED IMRT COMPARED TO MLC-BASED IMRT Dose in the buildup region wa s investigated for intensity-modulated radiation therapy (IMRT) delivery with solid brass modulat or and MLC. A Varian Clinac 2100 linear accelerator (LINAC) with MLC was used for b eam delivery. A solid brass step jig was designed and built to conduct IMRT test wi th compensator. Two step and shoot sequences were programmed to conduct IMRT test delivery with MLC. The profiles of the two delivery techniques were measured and adjusted to match at isocenter depth of 10 cm. Buildup dose at 1-5 mm dept h was measured with an ultr a-thin fixed volume parallel plate ionization chamber. Monte Carlo was used to model the brass step jig and step and shoot MLC sequences. The measured and simulated profiles for the two IMRT techniques were matched at isocenter de pth of 10 cm. Component doses including MLC component dose was calculated. Mean spect ral energy for open beam and compensated beam was calculated. (Intend to submit)

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78 5.1 Synopsis Dose in the buildup region of a 6MV Varian LINAC was compared for IMRT delivery with solid modulators and MLC. Near skin buildup dose of compensator treatment was a concern since compensator was closer to skin for Varian LINAC compared to existing machine. Dose was measured for 90 cm source to surface distance (SSD) and 106 cm SSD. The buildup depth was 1, 3, and 5 mm. Component dos es including MLC component dose was calculated. Mean spectr al energy was calculated for compensator. Buildup dose variation with SSD, field size an d beam incidence angle was calculated. The agreement between the measured and calc ulated IMRT profiles for compensator and MLC was 1.5 %. Key Words: Buildup dose, IMRT, step Ji g, Monte Carlo, compensator, MLC 5.2 Introduction Dose in the buildup region has been an area of interest in clinical radiation therapy even before 3D conformal therapy was introduced(58). The modeling of the dose in the buildup region when using intensity-modulated radiatio n therapy (IMRT) treatments has been the subject of radiation physics re search since the use of IM RT for head and neck cancer(59). Excessive dose in the buildup region can be the cause of significant patient discomfort and may lead to treatment interruptions, while underdosing in the buildup region may result in a local failure. Most commercial planning does not model dose in the buildup

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79 region well for small fields, such as the one s used in IMRT, and may not model the dose well with brass filters in the beam path. Many factors contribute to buildup dose, in cluding the photon beam energy spectrum, scattered or contaminant electrons and thei r angular distribution, and scattered low energy photons. Traditionally, these quanti ties are not modeled well in commercial treatment planning systems. As a result of th is deficiency, current commercial treatment planning systems have difficulties in calcula ting dose in buildup regions, as reported by Chung, et al.(60). These authors found that two TPS overestimated surface dose by 7.4% to 18.5%. The dosimetrical differences in buildup regions between different treatment modalities, such as MLC-based versus comp ensator-based IMRT, cannot be accurately obtained by comparing treatment plans. IMRT(61) planning uses invers e planning algorithms(62) that generate idealized fluence maps for each field. The fluence maps then mu st be converted to e ither a series of MLC segments that are deliverable. There are two techniques for using th e MLC to deliver the fluence maps: step and shoot, and sliding wi ndow. In the step and shoot approach, each segment is formed, then the beam is turned on for the appropriate number of monitor units (units of delivered radiation). Then th e beam is held off and the next segment is formed, and the process repeats. In the slid ing window approach, slit s of variable width move across the field and the dose rate is modulated.

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80 The fluence map for each field can also be ma de deliverable using custom compensators. The desired fluence is sent to a computer c ontrolled milling machine, which is used to mill out a solid brass plug to form a custom compensator for each field. The intensity is modulated by continuously varying the brass thickness. For each field, the associated compensator is placed in the beam path, typica lly in the wedge slot. Unlike the step and shoot delivery technique where only parts of th e field are exposed to radiation at a time, the entire field is exposed in a static fashion. Another major difference between the two IMRT techniques is that the compensator is always mounted closer to the patient than MLC is. This geometric difference could change the dose in the buildup region. This geometry difference is largely due to accelerator design. Of the di fferent LINAC designs, the Va rian LINACs mount puts the compensators closest to the patient and early studies in MLC sca tter and block scatter have shown that the closer the aperature, or in this case the compensator, the more scatter reaches the buildup region. However, compensators also filter out low energy photons and eliminate low energy electrons that are scattered from the LINACs collimation system. Measuring dose in the buildup region is by itsel f an area of research. There is an absence of electron equilibrium and the finite size of the fixed volume ionization chamber creates an uncertainty as to the ex act dose and the exact point of measurement. Although the extrapolation chamber, which allows one to va ry the chamber volume, is the detector of

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81 choice for buildup dose measurements, this de vice is bulky, time consuming to use, and is not available in many clinical sett ings, perhaps due to its limited use. Fixed volume parallel plate ionization chambers are widely used to measure superficial dose. Velkley et al(63) proposed corrections to reduc e the over-response of these chambers. The known over-response of these ch ambers at the surface has been compared with the extrapolation chamber and LIF ther mo luminescent detectors and the authors proposed guidelines in use of such detectors(64). Chamber specific correction factors have been reported by others (Rawlinson et al)(65). Rawlinson modified Velkley’s correction factor for fixed volume parallel plate ioniza tion chambers to include chamber geometry and wall material density. This correc tion factor was used in this study. Buildup dose was investigated for MLC-ba sed IMRT and compensator-based IMRT delivery with a Varian (Varian Medical Syst em Inc. Palo Alto, CA) linear accelerator (LINAC). To understand the differences in buildup doses be tween the delivery techniques, we commissioned a Monte Carl o code to calculate the buildup dose and component dose by components. So the ML C scatter dose photons electrons and the scatter from the compensator were all indivi dually evaluated. In addition, the mean spectral energy for compensators was calculate d. Finally, the total doses and component doses were compared for the two delivery techniques.

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82 5.3 Material and Methods 5.3.1 Study Setup A brass step modulator was designed and built to deliver a simple IMRT profile for a single field using a 6MV beam on a Varian 21 00. The simple design would allow for relatively straightforward mode ling of the resulting dose with Monte Carlo. To create a similar profile at 10 cm depth, using an MLC to modulate the beam, a series of equivalent MLC steps were also created for step and shoot delivery with Varian Millennium MLC. A commercial linear diode array (Sun Nucl ear Profiler, Sun Nuclear Corporation, Melbourne, FL) was set up to measure prof iles of compensator and MLC steps at isocenter depth of 10 cm. By adjusting the index of the MLC segments and adjusting the MLC shapes slightly, the MLC profiles were then matched to the profile of the brass compensator. Doses at a variety of depths in the buildup region were measured in solid water using an ultra-thin fixed volume paralle l plate ionization chamber connected to an electrometer. The MLC sequences and the brass modulator were modeled with Monte Carlo. The simulated parameters for the MLC and brass steps were adjusted to match their profiles to corresponding measured prof iles at 10 cm depth.

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83 5.3.2 Solid Brass Modulator The modulator is a 21x15 cm2 solid brass step as shown in Figure 23. It is mounted on an open port Plexiglas tray and inserted into th e head of the machine so that the proximal surface of the compensator is 57.6 cm from the source or 42.4 cm from the phantom surface. The distance from the distal surface of the compensator to the phantom surface varies from 41.8 to 35.4 (for 7 cm brass). Figure 23. Brass modulator mounted on an ope n port Plexiglas tray inserted into upper wedge slot of LINAC. From le ft to right side of the figur e, the step thickness is 7.62, 5.08, 4.0, 0.6, 1.0, 2.0 and 3 cm. Step width varied from 2.5 to 3. 4 cm projected at the isocenter. The effective Jaw setting was 21.4x15.4 cm2 at this plane.

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84 5.3.3 MLC Step and Shoot Sequences Test fields were designed using the Varian MLC SHAPER program version 6.2. Because of the field width, a spilt field IMRT prof ile with seven step and shoot segments was designed to mimic the dose profile from the compensator at 10 cm depth. The initial design of each segment used was 3x15 cm2. The initial dose fraction values for each segment in a sequence, was estimated using equations 4 and 5. BS 1i is the beam-ontime for segment i in sequence of the first field, and BS 2i is the beam-on-time for segment i in sequence in the second field. For each MLC segment in sequence, the corresponding brass step thickness is x1i and x2i, (the effective linear attenua tion coefficient) is 0.375 cm-1, N i i x i x ie e BS1 1 11 (4) N i i x i x ie e BSi1 2 22 (5) Weighted segment profiles were ad ded to obtain the MLC step profile.

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85 The segment weight SW 1i and SW 2i is calculated usi ng equations 6 and 7: 2 1 1 1 1 MU MU MU BS SWi i (6) 2 1 2 2 2 MU MU MU BS i SWi (7) MU 1 and MU 2 are the monitor units set on th e LINAC for sequence 1 and 2. N i N i i x i x N i i xe e e MUi11 2 1 1 11 (8) N i N i i x i x N j i xe e e MU11 2 1 1 22 (9) 5.3.4 Matching Profiles at 10 cm Depth Profiles of compensator steps and MLC segm ents were measured using a commercial linear diode array. The Profiler was set up so that the detector array was in a horizontal plane at 100 cm SSD (at isocenter), then 10 cm of solid water was placed on top of the Profiler. The profile generated from the comp enator was carefully measured. The profile generated from the MLC delivery initially did not match. The leaf positions for the MLC delivery were adjusted until the gradients in the profile of the MLC delivery matched that of the compensator. Then the dose index for the MLC segments was adjusted to match the MLC profile to the compensator profile.

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86 Finally the total number of monitor units was adjusted for the MLC delivery to deliver the same dose as the compensator plan. For this adjustment, an ion chamber replaced the Profiler in the phantom. In the end, 729 MUs (600+129) were delivered for the MLC sequences and 262 MUs for the compensator. 5.3.5 Chamber Measurements in the Buildup Region An ultra-thin window (0.02724 mm) fixed vol ume parallel plate ionization chamber (EXRADIN model A10, with the same collector diameter and electrode gap as Markus chamber) was connected to a MAX 4000 electrometer (Standard Imaging, Demingway WI). The chamber was set up in plastic water at 90 cm SSD. Chamber readings (nC) were obtained in the buildup region fo r individual slabs as well as for all steps of the two IMRT delivery techniques. The entire step and shoot sequence was run to obtain chamber response for each step. The chamber readings were corrected using Velkley’s correction factor which was modified by Rawlinson to account for geometry and wall material density, as in equation 10, ) max 4 ( 8 0* ) 1 ( ) ( ) ( ) (d de W E C d P d P (10) P/(d) is the corrected dose at depth d P(d) is measured dose at d the energy dependent factor C(E) is 27% for 6MV beam, l is plate separation, W is inner wall diameter, is wall material density, d is the depth to front surface of chamber, and dmax is the depth of maximum dose respectively.

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87 5.3.6 Monte Carlo Modeling An EGSnrc(7)-based Monte Carlo simulation packag e for clinical radiation treatment units, BEAMnrc(34), was used to simulate the radiati on from the Varian linear accelerator using either an MLC or brass compensato rs. The Varian Trilogy linear accelerator was first modeled for just open fields and tested against measured depth doses and dose profiles in a water phantom. Once this was completed a phase space file, in which are stored physical parameters (such as charge energy, position, a nd direction) for all particles (photons and electrons ) traversing the plane of in terest, was scored below the secondary jaws. A total of 5x108 electrons with kinetic energy of approximately 6MeV incident on the target, were simulated to generate a phase space file. The phase space files were then used as radiation sources for MLC and compensator step simulations. The phase space files from MLC and compensator step simulations were used as radiation sources in phantom dose di stribution calculations using DOSXYZnrc(13) (DOSXYZ user’s manual nrc report). In the DOSXYZ program the phantom geometry, compensator design, and, individual ML C profiles were defined. Water phantom simulations were done at 90 cm and 106 cm SSD. The buildup dose was calculated in components: the primary photon dose, the scattered radiation dose and electron contaminant dose. The energy fluence distribution for open beams and with a 2 cm thick slab of brass in the beam were calculated to show the effect of beam hardening with compensators.

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88 The physical width and dimension of the br ass steps were carefully measured and modeled using the component module BLOCK in which the compensator divergence was accounted for. For the MLC, each segment dimension was adjusted and simulated accordingly. The accelerator models are show n in Figure 24 (a, b). The weight for each MLC segment were defined by the monitor units delivered in each segment divided by the total number of MUs used to deliver the MLC pattern. Figure 24. Monte Carlo model of Varian accelerator head geometry. Major parts such as the target, primary collimator, flattening filter, transmission chamber and jaws are shown in panel a. MLC, the step wedge and the phant om top is shown in panel b. The measured and calculated doses were normalized to th e calculated 10 cm depth point along the central axis. Buildup dose at shallow depths were then ca lculated with Monte Carlo. The depth dose in the buildup region along the IMRT beam axis was calculated. The percent of dose difference of the two IMRT techniques in the buildup region as a function

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89 of step modulation was measured. Depth dose for individual slabs of solid brass was calculated as a function of brass thickness, field size, and source to surface distance. Energy spectral variation for open and compensated field was calculated. The contaminant dose component in the buil dup region was calculated. Buildup dose contributed from photons and el ectrons that were interacted in MLC were calculated. 5.3.7 Dose in Buildup Versus Source Surface Distance Dose at 1.0 mm depth with extended SSD was also calculated. This is done to approximate the dose difference with a diff erent SSD which may occur with varying patient treatment techniques and also with di fferent accelerator geom etries if a linear accelerator other than a Varian is used. This only approximates the differences one would see with a different accelerato r, as a complete simulation of the other accelerator would be required. To approximate the effect due to the modulator to surface distance change, but not SSD change, all the results from both measurem ents and calculations were corrected using Mayneord’s F factor.

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90 5.4 Results 5.4.1 Dose Profile Match at 10 cm Depth Matched profiles of solid modulator and step and shoot MLC at isocenter depth of 10 cm is shown in Figure 25. Agreem ent between the measured and calculated profiles was better than 1.5%. Figure 25. The matched profiles of IMRT delivery with step and shoot and solid modulator. Center of 6MV beam traverses the thinnest part of the modulator which is 0.6 cm thick brass.

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91 5.4.2 Dose Comparison at Shallow Depths Agreement between the Monte Carlo and measur ed data of 3 % or better was achieved for the more clinically relevant IMRT steps of (1, 2 and 3 cm thick) at 1, 3 and 5 mm depth in reference to 10 cm dept h, as in Tables 6, 7 and 8. The measured and calculated dose in cGy (giv en 100 cGy delivered at isocenter) for the most commonly used clinical compensator with equivalent step thickness of 2 cm, were 47 and 44.01.0 for compensator compared to 52 and 49.01.5 for MLC at 1 mm depth in Table 6. Table 7 shows the same values for 3 mm depth were 71 and 70.01.8 for compensator compared to 75 and 76.02.3 for MLC. The same values at 5 mm depth were 83 and 82.01.6 for compensator and 90 and 89.02.3 for MLC in Table 8. Tables show the same trend of lower compensator dose compared to MLC for more clinically relevant 1 cm and 3 cm thick equivalent steps. Table 6. Doses (cGy) of IMRT delivery with solid modulator and MLC as a function of STEP thickness of the compensator at 1 mm de pth. Doses were calculated with Monte Carlo and measured with parallel plate i onization chamber at 90 SSD, 0.1 cm depth and normalized to dose at isocenter, assuming 100 cGy was delivered at the isocenter. STEP cm COMP MLC Calculated Measured Calculated Measured 0.6 69.01.0 69 71.01.4 70 1.0 62.01.0 64 66.02.0 68 2.0 44.01.0 47 49.01.5 52 3.0 30.01.0 30 36.01.1 34 4.0 27.01.0 29 29.01.0 31 5.08 20.01.0 21 22.01.1 24 7.62 13.01.0 12 13.01.0 13

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92 Table 7. Doses (cGy) of IMRT delivery with solid modulator and MLC as a function of STEP thickness of the compensator at 3 mm de pth. Doses were calculated with Monte Carlo and measured with parallel plate i onization chamber at 90 SSD, 0.3 cm depth and normalized to dose at isocenter, assuming 100 cGy was delivered at the isocenter. STEP cm COMP MLC Calculated Measured Calculated Measured 0.6 113.01.7 109 116.02.3 113 1.0 100.02.0 98 104.02.1 102 2.0 70.01.8 71 76.02.3 75 3.0 50.01.5 48 55.01.7 53 4.0 39.01.2 39 41.01.2 43 5.08 27.01.1 27 31.01.2 31 7.62 15.01.0 15 16.01.0 15 Table 8. Doses (cGy) of IMRT delivery with solid modulator and MLC as a function of STEP thickness of the compensator at 5 mm de pth. Doses were calculated with Monte Carlo and measured with parallel plate i onization chamber at 90 SSD, 0.5 cm depth and normalized to dose at isocenter, assuming 100 cGy was delivered at the isocenter. STEP cm COMP MLC Calculated Measured Calculated Measured 0.6 136.02.0 133 138.02.8 135 1.0 118.01.8 119 123.02.8 122 2.0 82.01.6 83 89.02.3 90 3.0 58.01.5 57 62.01.9 60 4.0 45.01.1 46 47.01.6 49 5.08 32.01.1 32 34.01.4 35 7.62 17.01.0 16 17.01.0 16 Figure 26 shows percent of dose difference as a function of step modulation for IMRT with MLC versus compensator. The differe nce increased with compensator thickness reaching the maximum value of 17% for the equi valent of 5 cm thick brass step at 1 mm

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93 depth dropping to 8% at 5 mm depth. It decr eased with further incr ease in modulation or brass thickness, and depth. Figure 26. Percent of dose difference for IMRT test delivery with MLC with respect to compensator is shown as a function of step modulation (thickness) for 1, 3 and 5 mm depth. Note the maximum difference occurs for the 5 cm thick step for all depths and is highest for 1 mm depth. The value is about 17%. 5.4.3 Shallow Dose Variation with SSD Figure 27 shows dose variation with SSD. Dose at 1 mm depth for open beam and beam attenuated with 1 cm brass was 71 and 64 and did not chan ge with SSD; however. dose for 5.08 cm brass decreased 10.5% from 56.0 to 45.5 for 90 to 106 cm SSD.

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94 5.4.4 Shallow Dose Variation with Field Size Figure 28 shows dose variation with field si ze. Dose increased 10 percentage points (67% to 77%) for open beam of (5x5-20x20) cm2. Dose increase was 12 percentage points (58% to 70%) for 1 cm brass and 25 percentage points (43% to 68%) for 5.08 cm brass. Figure 27. Buildup dose variation with SSD for open and compensated fields. Note the bottom two curves show the largest variati on in buildup dose with SSD for the 5.08 cm thick compensator.

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95 Figure 28. Dose variation w ith field size. Dose variati on for 2, 5, 10, 15 and 20 cm2 fields is presented for open and co mpensated fields at 1 mm depth. 5.4.5 Energy Spectra Variation for Open and Compensated Field The average spectral energy of photons was 1.57 MeV for open beam and 2.17 MeV for 2 cm thick brass, as shown in Figure 29.

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96 Figure 29. Normalized plan ar energy fluence distribution of 6MV beam for 2x15 cm2 field.

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97 5.4.6 Dose Contributions of Various Components Calculated Component doses of solid modulator and step and shoot MLC as percentage of total dose is shown in Table 9. Table 9. Monte Carlo percent of tota l dose contribution from scattered photons, contaminant electrons and MLC. Dose at 1.0 mm depth was scored in a 1.5 cm2 square region centered on the central axis. Se t up SSD was (a)=90 cm and (b)=106 cm. IMRT Total dose cGy Scattered Photons % Contaminant Electrons % MLC Component % Solid modulator (a) 69.01.0 81 19 0.08 Step and shoot (a) 71.03.1 82 15 3.0 Solid modulator (b) 50.01.0 82 18 0.08 Step and shoot (b) 54.02.7 80 18.0 2.0

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98 5.4.7 Scatter Photon Dose Contribution The primary photons are those generated in the target and pass di rectly through the flattening filter reaching the phantom surface on the axis with a narrow angular spread and bear minimal scatter component. The contaminant photons or electrons have interacted in and scattered fr om accelerator head components such as the jaws or MLC before reaching the phantom surface. These components were differentiated in pha ntom dose calculation in BEAMnrc. For example, the latch bit filtering in BEAMnrc was used to tag the scattered dose from the MLC. This was accomplished by setting bit 5, for example, in the MLC component module to be associated with this region fo r use with the bit filters applied when dose components are used. Total dose and dose compon ents were selected for dose calculation in BEAMnrc input file. The particles that were interacted and scattered from MLC entering the phantom were defined to be cont aminant. The contaminant type was set to either photon or electron. Finally, the num ber of dose components inclusive bit filters was defined to include the associated com ponent latch bit filters for component dose calculation. Low energy scatter photon dose contribution at 1.0 mm depth was 82% of total dose for compensator and 84% of total dose for step a nd shoot delivery at 90 cm SSD. At 106 cm SSD, these values did not change signifi cantly and were 84% of total dose for compensator and 80% of total dose for step a nd shoot delivery tec hniques respectively.

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99 5.4.8 Contaminant Electron Dose Contribution Contaminant electron dose contribution was significantly lower compared to that from scattered photons at 1 mm depth. Dose cont ribution at 90 cm SSD was 18% of total dose from solid modulator compared to 12% of to tal dose from step and shoot delivery. The corresponding values at 106 cm SSD were 16% and 18% of tota l dose respectively. Our preliminary calculations in the buildup region at shallo wer depths near the skin showed low energy contaminant electrons and ph otons contributed nearly equally to total dose. However, the focus of this study wa s to assess buildup dose between 1-5 mm depth. 5.4.9 MLC Component Dose Contribution MLC scatter contribution to total dose was le ss than 0.1% for solid modulator and 2% for step and shoot delivery. MLC component dose c ontribution was about 4% of total dose at shallow depths at 90 cm SSD and 2% at 106 cm SSD respectively. 5.5 Discussion The results presented here were limited to simple IMRT tests conducted for the step jig device, however the conclusions that were drawn can be extended to the actual IMRT plans. The magnitude of this effect is simila r to the effect we woul d expect to see with

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100 large field IMRT plans where the smallest segments are no wider than 2 cm. In cases with much higher degrees of modulati on, the MLC scatter would increase. Usually in treatment planning system calcu lations, the voxel size is 2-3 mm, and TPS dose calculation uncertainty greatly increa ses at shallower depths. The dosimetric uncertainty in this region is also pronounced even with M onte Carlo calcul ations. As the simulation thickness becomes smaller to acco mmodate calculations at shallower depths, the number of particles interact ing in such a thin slab is extremely sparse, hence to perform a calculation with small uncertainty a prohibitively large number of histories would be required to achieve the requisite statistics. Also, in a separate but relate d study, Opp et al. analyzed pr eviously planned cases using IMRT with solid compensatrors. In this work, a histogram of transmission factors (plotted by compensator thickness) for 10 cases (with a total of 50 brass modulators) was generated, it is clear that the most pr obable compensator thickness as weighted by transmission factor (or dose) was just under 2 cm. as shown in Figure 30.

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101 Figure 30. Most probable compensator thickness from 50 retrospective IMRT field analysis (Opp et al.). The step wedge IMRT simulation calculatio ns for Varian LINAC show dose in the buildup region at depth of 5 mm is 7% lower for compensators with equivalent thickness of 2 cm modulation, compared with MLC. This dose reducti on is due to beam hardening of compensators. We conclude that dose at 5 mm depth in the buildup region is 7% lower for compensatorbased IMRT compared with MLC-based IMRT delivery on the 6MV Varian Trilogy linea r accelerator. Simulation data indicate dose contribution from MLC at 1 mm depth was less than 3% of total skin dose for step and shoot test delivery. This is partly due to transmission of useful beam through closed MLC leaves (~ 2% for Va rian MLC) and partly due to contaminant

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102 radiation emitted from interaction of primary beam with the edges of open MLC segments. 5.5.1 Measured Dose Gradient Note that making measurements in the buildup region are difficult due to steep dose gradients. The measured dose varied 17%/mm between depth of 2 and 3 mm and 10%/mm between 4 and 5 mm depth. Acute skin toxicity with step and shoot IMRT for head and neck has been attributed to many factors, including multiple tangential beams, the beam obliquity and the bolus effect of immobilization mask fo r the head and neck treatments(66). Authors indicated the average skin dose was 18% larger than the tr eatment planning system indicated, due to a variety of effects, includi ng the bolusing effect of the mask and beam obliquity. 5.6 Conclusion Low energy scattered electrons emerging from accelerator head for the step and shoot and brass modulator delivery were major cont ributors to dose in the buildup region near the surface, which sharply decreased in magn itude while that of photons increased with depth beyond 1 mm, however contaminant c ontribution from the photons scattered off the MLC increased over the first few millimeters in the buildup region.

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103 The closer the modulator is to the patient, th e higher skin dose the patient would get, due to the larger solid angle to the scattered low energy photons and electrons from the accelerator head. In all accelerator configurations, the brass compensator is always closer to the patient than the MLC is. However, Mo nte Carlo simulations in this study indicate that compensator dose was 5%, 6% and 7% lower than MLC dose at 1 mm, 3 mm and 5 mm depth for modulation equivalent to 2 cm of brass. Beam hardening in the compensator modality is the major reason for its shallow-depth dose sparing in the buildup region. Further extended SSD measur ements and simulations in this study suggest that the use of accelerators other th an Varian’s, which usually have smaller heads, thus larger modulator to surface di stance, would further reduce the dose in the buildup region when using compensators.

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104 CHAPTER 6 CONCLUDING REMARKS The dose in the first few millimieters of the build up region is most strongly influenced by beam spectrum and the SSD. The more th e engery spectrum is shifted to higher energy the lower the dose in the buildup region. Similarly the greate r the SSD the more the dose in the buildup region is reduced. In the case of compensators the spectrum is shifted to higher e ngeries due to beam hardening, and the compensator also filter s out the low energy scattered photons and electrons resulting in lower dose in first few millimeters of the the buildup region. In the case of the Tomotherapy unit the beam has no flattening filter resulting in an energy spectrum that is shifted to lower ener gies, and the treatment s use a smaller SSD. Also the beam angles played a role as b eams from shallow angles have longer path lengths. 6.1 Recommendations for Future Work In radiation oncology there are times when getting a higher dose in the buildup region is desirable, such as in the post-operative sett ing where cancer cells may have seeded along the surgical scars. In other cases this hi gh dose in the buildup region can cause unwanted

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105 skin reaction and breaks in a patient’s tr eatment. However there is precious little knowledge of exactly what dose to what depth will cause the desired or undesired effects. A clinical investigation may be able to address this issue. The measurements in the first 4 mm of the buildup region in our work have significant uncertainty. Repeating these measurements with an extrapolation chanber may result in much more accurate measurements which we believe would be in better agreement with our Monte Carlo results.

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106 REFERENCES 1. Leung.Phillip.M.K., Sontag.Marc.R., Maharaj.Harrideo et al Dose measurements in the buildup region for Cobalt-60 therapy units. Med Phys 1976:3. 2. Taylor.A., Powell.M.E.B. Intensitymodulated radiotherapy-What is it? Cancer Imaging 2004; 4(2):68-73. 3. Fraas.B., Doppke.K., Hunt et al American Association of Physicists in Medicine Radiation Therapy Committee Task Group 53: Quality assurance for clinical radiotherapy treatment planning. Med Phys 1998; 25:1773-1829. 4. Bielajew.A.F., Rogers.D.W.O., Nahum .A.E. Monte Carlo simulation of ion chamber response to 60Co-resolution of anomolies associated with interfaces. Phys Med Biol 1985; 30:419-428. 5. Mackie.T.R., Rocwerdt.P.J., Wells.C.M. et al The OMEGA project: comparison among EGS4 electron beam simulations, 3D Fermi-Eyges calculations, and dose measurements Proc. 11th Int. Conf. on the Use of Computers in Radiation Therapy ed A.R. Hounsell, J.M. Wilkins on and P.C. Williams (Manchester: North Western Medical Physics Department, Ch ristie Hospital NHS Trust). 1994. 6. Mohan.R. Why Monte Carlo? Proc. 12th Int. Conf. on the Use of Computers in Radiation Therapy ed D.D. Leavitt and G. Starkschall (Salt Lake City, UT), Madison, WI:Medical Physics Publishing. 1997:16-18. 7. Kawrakow.I. Accurate condensed hist ory Monte Carlo simulation of electron transport. I. EGSnrc, the new EGS4 version. Med Phys 2000; 27:485-498. 8. Kawrakow.I., Rogers.D.W.O. The EGSnrc code system: Monte Carlo simulation of electron and photon transport techni cal report RIRS-701 (Ottawa, Canada: National Research Council of Canada). 2000. 9. Rogers.D.W.O., Walters.B., Kawrakow .I. BEAMnrc users manual NRC Report PIRS 509(a)revH, ( http://irs.inms.nrc.ca/software/beamnrc/ ). 2004. 10. Ding.G.X. Energy spectra, angular spr ead, fluence profiles a nd dose distributions of 6 and 18 MV photon beams: results of Monte Carlo simulations for a Varian 2100EX accelerator Phys Med Biol 2002; 47:1025-1046.

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107 11. Sheikh-Bagheri.D., Rogers.D.W.O. Se nsitivity of megavol tage photon beam Monte Carlo simulations to electron beam parameters. Med Phys 2002; 29:379390. 12. Verhegaegen.F., Seuntjens.V. Monte Ca rlo modeling of external radiotherapy photon beams. Phys Med Biol 2003; 48:R107-R164. 13. Ma.C.M., Reckwerdt.P., Holmes.M. et al. DOSxyz users manual NRC report. National Research Council of Canada. 1995. 14. Ma.C.M., Rogers.D.W.O. "BEAMDP User s Manual" National Research Council of Canada Report PIRS-0509D (NRC, Ottawa). 1995. 15. Javedan.Khosrow, Stevens.Craig.W, Forster.Kenneth.M. Compensator-based intensity-modulated radiation therapy fo r malignant pleural mesothelioma post extrapleural pneumonectomy. Journal of Applied Clin ical Medical Physics 2008; 9:98-109. 16. Aref.A., Thornton.D., Youssef.E., et al. Dosimetric improvements following 3D planning of tangentia l breast irradiation. Int J Radiat Oncol Biol Phys 2000; 48:1569-1574. 17. Hansen.V.N., Evans.P.M., Shentall.G.S. et al. Dosimetric evaluation of compensation in radiotherapy of the breast: MLC intensity modulation and physical compensators. Radiother Oncol 1997; 42:249-256. 18. Chao.K.S.C., Perez.C.A., Brady.L.W. Breast: locally advanced (T3 and T4), inflammatory, and recurrent tumors. Radiation oncology management decisions 2002:367-375. 19. Hong.L., Alektiar.K., Chui.C., et al. Imrt of large fields: whole-abdomen irradiation. Int J Radiat Oncol Biol Phys 2002; 54:278-289. 20. Ragaz.J., Jackson.S.M., Le.N., et al. Adjuvant radiotherapy and chemotherapy in node-positive premenopausal women with breast cancer. N Engl J Med 1997; 337:956-962. 21. McCormick.B.M., Hudis.C., Dershaw.D.D ., et al. Breast cancer, in Principles and practice of gynecologic oncology, edited by W.J. Hoskins, C.A. Perez, R.C. Young, R.R. Barakat and M. Markma n. Lippincott Williams & Wilkins, Philadelphia. 2004:1077-1170. 22. Wells.M., Macmillan.M., Raab.G., et al. Does aqueous or sucralfate cream affect the severity of erythematous radiati on skin reactions? A randomised controlled trial. Radiother Oncol 2004; 73:153-162.

PAGE 121

108 23. Quach.K.Y., Morales.J., Butson.M.J., et al. Measurement of radiotherapy x-ray skin dose on a chest wall phantom. Med Phys 2000; 27:1676-1680. 24. Kron.T., Grigorov.G., Yu.E., et al Planning evaluation of radiotherapy for complex lung cancer cases us ing helical tomotherapy. Phys Med Biol 2004; 49:3675-3690. 25. Sheng.K, Molley.J.A, Larner.J.M., et al. A dosimetric comparison of noncoplanar IMRT versus Helical TomoThera py for nasal cavity and paranasal sinus cancer. Radiother Oncol 2007; 82:174-178. 26. Cheek.D., Gibbons.J.P., Rosen.I.I. et al. Accuracy of TomoTherapy treatments for superficial target volumes. Med Phys 2008; 35:3565-3573. 27. Ramsey.C.R., Seibert.R.M., Robison.B., et al. Helical tomotherapy superficial dose measurements. Med Phys 2007; 34:3286-3293. 28. Hardcastle.N., Soisson.E., Metcalfe.P. et al. Dosimetric ve rification of helical tomotherapy for total scalp irradiation. Med Phys 2008; 35:5061-5068. 29. The biological basis for dose limitation in the skin. ICRU 1992. 30. Chow.J.C.L., Grigorov.G.N. Surface dosimetry for oblique tangential photon beams: a Monte Carlo simulation study. Med Phys 2008; 35:70-76. 31. Lu.W., Olivera.G.H., Chen.M-L. et al. Accurate convol ution/superposition for multi-resolution dose calculation using cumulative tabulated kernels. Phys Med Biol 2005; 50:655-680. 32. Evans.P.M., Donovan.E.M., Partridge.M ., et al. The delive ry of intensity modulated radiotherapy to the brea st using multiple static fields. Radiother Oncol 2000; 57:79-89. 33. Battista.J.J., Sharpe.M.B. True three-dimensional dose computations for megavoltage x-ray therapy: A role for the superposition principle. Australas Phys Eng Sci Med 1992; 15:159-178. 34. Rogers.D.W.O., Faddegon.B.A., Ding.G.X ., et al. BEAM: A Monte Carlo code to simulate radiotherapy treatment units. Med Phys 1995; 22:503-524. 35. Fenwick.J.D., Tome.W.A., Jaradat.H.A., et al. Quality assurance of a helical tomotherapy machine. Phys Med Biol 2004; 49:2933-2953.

PAGE 122

109 36. Tournel.K., Verellen.D., Duchateau, et al. An assessment of the use of skin flashes in helical tomotherapy using phantom and in-vivo dosimetry. Radiother Oncol 2007; 84:34-39. 37. Xiang.H.F., Song.J.S., Chin.D.W.H., et al. Build-up and surface dose measurements on phantom using microMOSFET in 6 and 10 MV x-ray beams and comparisons with Monte Carlo calculations. Med Phys 2007; 34:1266-1273. 38. Bang.K.M., Pinheiro.G.A., Wood.J.M. et al. Malignant mesothelioma mortality in the United States, 1999-2001. Int J Occup Environ Health 2006; 1:9-15. 39. Porta.C., Ardizzoni.A., Gaudino.G. et al. Malignant mesothelioma in 2004: how advanced technology and new drugs ar e changing the perspective of mesothelioma patients. Med Lav 2005; 96(4):360-369. 40. Price.B., Ware.A.. Mesothelioma trends in the United States: an update based on surveillance, epidemiology, and end results program data for 1973 through 2003. Am J Epidemiol 2004; 159(2):107-112. 41. Kutcher.G.J., Kestler.C., Greenblatt.D. et al. Technique for external beam treatment for mesothelioma. Int J Radiat Oncol Biol Phys 1987; 13(11):17471752. 42. Soubra.M., Dunscombe.P.B., Hodson.D.I., et al. Physical aspects of external beam radiotherapy for the treatment of malignant pleural mesothelioma. Int J Radiat Oncol Biol Phys 1990; 18:1521-1527. 43. Hilaris.B.S., Non.D., Kwong.E., et al. Pleurectomy and interoperative brachytherapy and post operative radiation in the treatment of malignant pleural mesothelioma. Int J Radiat Oncol Biol Phys 1984; 10:325-331. 44. Bissett.D., Macbeth.F.R., Cram.I., et al. The role of palliative radotherapy in malignant mesothelioma. Clin Oncol (R Coll Radiol) 1991; 3(6):315-317. 45. de.Graaf-Strukowska.L., Van.der.Zee.J., Van.Putten.W., et al. Factors influencing the outcome of radiotherapy in malignant mesothelioma of the pleura a singleinstitution experience with 189 patients. Int J Radiat Oncol Biol Phys 1999; 43(3):511-516. 46. Yajnik.S., Rosenzweig.K.E., Mychalczak.B ., et al. Hemithorasic radiation after extrapleural pneumonectomy for ma lignant pleural mesothelioma. Int J Radiat Oncol Biol Phys 2003; 56(5):1319-1326. 47. Meeks.S.L., Buatti.J.M., Bova.F.J., et al Potential clinical efficacy of intensitymodulated conformal therapy. Int J Radiat Oncol Biol Phys 1998; 40:483-495.

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110 48. Ahmad.A., Stevens.Craig.W., Smythe.Roy.W., et al Promising Early Local Control of Malignant Pleural Mesothel ioma Following Postoperative Intensity Modulated Radiation Therapy (IMRT) to the chest. The cancer journal 2003; 9. 49. Forster.K.M., Smythe.R.W., Starks hall.G., et al. Intensity-Modulated Radiotherapy Following Extrapleural Pneumonectomy for the Treatment of Malignant Mesothelioma: Clinical Implementation. 1: Int J Radiat Oncol Biol Phys 2003; 3:606-616. 50. Rice.D.C., Stevens.C.W., Correa.A.M. et al. Outcomes after extrapleural pneumonectomy and intensity-modulated radi ation therapy for malignant pleural mesothelioma Ann Thorac Surg 2007; 84(5):1685-1692. 51. Malhorta.H.J., Raina.S., Avadhani.J.S. et al. Technical and dosimetric considerations in IMRT treatment planning for large target volumes. Journal of Applied Clinical Medical Physics 2005; 6. 52. Chang.S.X., Cullip.T.J., Deschesne.K. M., et al. Intensity modulation delivery techniques: “step and shoot” MLC auto-s equence versus the use of modulator. Med Phys 2000; 27:948-959. 53. Chang.S.X., Cullip.T.J., Deschesne.K.M. et al. Compensators: an alternative IMRT delivery technique. J Appl Clin Med Phys 2004; 5(3):15-36. 54. DuPlessis.F.C.P., Willemse. C.A. Monte Carlo calculation of effective attenuation coefficients for various compensator materials. Med Phys 2003; 30 (9). 55. Jiang.S.B., Ayyangar.K.M.. On compensator design for photon beam intensitymodulated conformal therapy. Med Phys 1998; 25 (5). 56. Van't.Reit.A., Mak.A.C., Moerland.M.A., et al A conformation number to quantify the degree of conformity in Brac hytherapy and external beam irradiation: Application to the prostate. Int J Radiat Oncol Biol Phys 1997; 37:731-736. 57. Allen.A.M., Xzerminiska.M., Janne.P.A., et al. Fatal pneumonitis associated with intensity-modulated radiation therapy for mesothelioma. Int J Radiat Oncol Biol Phys 2006; 65(3):640-645. 58. Slobodan.Devic, Gyorgy.Hegyi, Te.V uong, et al. Comparative skin dose measurement in the treatment of anal cancer: Conventional versus conformal therapy. Med Phys 2004; 31(6):1316-1321.

PAGE 124

111 59. Paelink.L., Wagter.C.D., Esch.A.V., et al. Comparison of buildup dose between Elekta and Varian linear accelerators for high-energy photon beams using radiochromic film and clinical implicatio ns for IMRT head and neck treatments. Phys Med Biol 2005; 50(3):413-428. 60. Chung.H., Jin.H., Demsey et al Evaluation of surface and buildup region dose for intensity-modulated radiation therapy in head and neck cancer. Med Phys 2005; 32:2682-2689. 61. Webb.S. The physical basis of IMRT and inverse planning. The British Journal of Radiology 2003; 76:678-689. 62. Xing.L., Hamilton.R.J., Spelbring.D. et al. Fast iterative algorithms for threedimensional inverse treatment planning. Med Phys 1998; 25(10):1845-1849. 63. Velkley.D.E., Manson.D.J., Purdy.J.A. Build-up region of Me gavoltage radiation sources. Med Phys 1975 ;2:14-19. 64. Gerbi.B.J., Khan.F.M. Measurement of dose in the buildup region using fixedseparation plane-parallel ionization chambers. Med Phys 1990; 17:17-26. 65. Rawlinson.J.A., Arlen.D., Newcombe.D. Design of parallel-plate ion chambers for buildup measurements in megavoltage photon beams. Med Phys 1992; 19:641-648. 66. Lee.N., Chuang.C., Qujyey.J.M., et al. Sk in toxicity due to intensity-modulated radiotherapy for head-and-neck carcinoma. Int J Radiat Oncol Biol Phys 2002; 53:630-637. 67. Stevens.C.W., Wong.P.F., Rice.D., et al.. Treatment planning system evaluation for mesotheliomaIMRT. Lung Cancer 2005; 49(Suppl 1):S75–S81.

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ABOUT THE AUTHOR Khosrow Javedan lives with his wife Michel e Javedan and their son Anthony R. Javedan in Tampa Bay area. He completed his Bach elor’s degree at Albion College, Albion Michigan in 1981. He worked at William Beau mont Hospital as an engineer and started his graduate studies in an accredited medical physics program at Wa yne State University in Detroit, Michigan and received his Mast er’s degree in medical physics in 1993. He joined Georgetown University hospital work ing in radiation oncology department as junior physicist and after 2 ye ars as clinical physic ist in 1996. He joined Massachussetts General Hospital in 1999 while attending gradua te school at University of Lowell. He became board certified in therapeutic medi cal physics from the American Board of Medical Physics in 1999 and fr om American Board of Radiology in 2000. He joined the H. Lee Moffitt Cancer Center as a clinical radiation ph ysicist in 2003 and started pursuing his Ph.D. at the Universi ty of South Florida in 2004.