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A Dissertation Entitled Investigation of Radiation Protection Methodologies for Radiation Therapy Shielding Using Monte Carlo Si

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A Dissertation Entitled Investigation of Radiation Protection Methodologies for Radiation Therapy Shielding Using Monte Carlo Si A Dissertation entitled Investigation of Radiation Protection Methodologies for Radiation Therapy Shielding Using Monte Carlo Simulation and Measurement by Sean Tanny Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Doctor of Philosophy De- gree in PhysicsWith a Concentration in Radiation Oncology Physics Dr. E. Ishmael Parsai, Committee Chair Dr. David Pearson, Committee Member Dr. Diana Shvydka, Committee Member Dr. Jon Bjorkman, Committee Member Dr. Richard Irving, Committee Member Dr. Patricia R. Komuniecki, Dean College of Graduate Studies The University of Toledo December 2015 Copyright 2015, Sean Tanny This document is copyrighted material. Under copyright law, no parts of this document may be reproduced without the expressed permission of the author. An Abstract of Investigation of Radiation Protection Methodologies for Radiation Therapy Shielding Using Monte Carlo Simulation and Measurement by Sean Tanny Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Doctor of Philosophy De- gree in PhysicsWith a Concentration in Radiation Oncology Physics The University of Toledo December 2015 The advent of high-energy linear accelerators for dedicated medical use in the 1950's by Henry Kaplan and the Stanford University physics department began a revolution in radiation oncology. Today, linear accelerators are the standard of care for modern radiation therapy and can generate high-energy beams that can produce tens of Gy per minute at isocenter. This creates a need for a large amount of shielding material to properly protect members of the public and hospital staff. Standardized vault designs and guidance on shielding properties of various materials are provided by the National Council on Radiation Protection (NCRP) Report 151. However, physicists are seeking ways to minimize the footprint and volume of shielding material needed which leads to the use of non-standard vault configurations and less-studied materials, such as high-density concrete. The University of Toledo Dana Cancer Center has utilized both of these methods to minimize the cost and spatial footprint of the requisite radiation shielding. To ensure a safe work environment, computer simulations were performed to verify the attenuation properties and shielding workloads produced by a variety of situations where standard recommendations and guidance documents were insufficient. This project studies two areas of concern that are not addressed by NCRP 151, the ra- iii diation shielding workload for the vault door with a non-standard design, and the attenuation properties of high-density concrete for both photon and neutron radia- tion. Simulations have been performed using a Monte-Carlo code produced by the Los Alamos National Lab (LANL), Monte Carlo Neutrons, Photons 5 (MCNP5). Mea- surements have been performed using a shielding test port designed into the maze of the Varian Edge treatment vault. iv Acknowledgments I would like to acknowledge the Department of Radiation Oncology at the University of Toledo Medical Center for the phenomenal opportunities they have afforded me in my training and research. We always discuss the quality of the education that students receive within our program, and as I continue to discover other programs, the more I am convinced of the quality of the education I received at the hands of some of the most dedicated professionals I've met. Thank you to Dr. Shvydka for tolerating the noise I can make when confused. Thank you to Dr. Pearson for always offering sound advice, whether it be about car repair or complicated clinical measurements. Thank you Dr. Sperling for the camaraderie, sharing your scientific and technical expertise so freely, and all the great collaborative projects we've been able to produce. I owe so much to Dr. Parsai for the faith he's shown in me. I certainly would not be the person I am today without his guidance and constant patience. I want to thank Dan Harrell, Jim Noller, Brett Dietrich, and Manjit Chopra from Shielding Construction Solutions and Universal Minerals International for shar- ing their dedication, knowledge, and experience in material mixtures and radiation shielding design. Without their insight and contributions, the unique design of the Dana Cancer Center and the shielding test port would never have come to fruition. No graduate student is capable of navigating the complicated bureaucracy between two colleges on two separate campuses without the guidance and experience of the supporting staff. I want to acknowledge and thank Diane Adams, Lynda Obee, and v Tiffany Akeman for their patience in explaining the necessary forms and pointing me in the right direction to make sure I can continue on in my research. Your help has been invaluable. Finally, this dissertation would still be in disarray if it weren't for the dedicated efforts of my wife, Kristen. Thank you so much for being there and tolerating all the late nights and weekends it has taken to finish this work. vi Contents Abstract iii Acknowledgments v Contents vii List of Tables xi List of Figures xiii List of Abbreviations xv Preface xvii 1 Introduction 1 2 Radiation Protection and Radiation Units 6 2.1 Radiation Units . 8 2.1.1 Exposure . 9 2.1.2 Dose . 9 2.1.3 Equivalent Dose . 10 2.1.4 Effective Dose . 11 2.1.5 Ambient Dose Equivalent . 12 2.2 Dose Limits for Radiation Protection . 13 2.2.1 Time-Average Dose Rate Limits . 15 vii 2.3 Radiation Interactions with Matter and Exponential Attenuation . 16 2.3.1 Radiation Interactions with Matter . 16 2.3.1.1 Photoelectric Absorption . 16 2.3.1.2 Compton Scatter . 18 2.3.1.3 Pair/Triplet Production . 19 2.3.2 Attenuation Coefficients . 19 2.3.3 Exponential Attenuation . 20 3 NCRP Report 151 22 3.1 Primary and Secondary Barriers . 22 3.2 Occupancy Factors . 25 3.3 Use Factors . 27 3.4 Accelerator Workload . 28 3.4.1 Primary Workload . 28 3.4.2 Leakage Workload . 29 3.4.3 Patient-Scatter Workload . 30 3.5 Determining Barrier Thicknesses . 31 3.6 Door Shielding . 34 3.6.1 Photon Dose-Equivalent at the Door . 35 3.6.1.1 Photon Dose-Equivalent for Machines Producing ≤10 MV Photon Beams . 36 3.6.1.2 Photon Dose-Equivalent for Machines Producing >10 MV Photon Beams . 38 3.6.2 Neutron Dose-Equivalent at the Maze Door . 40 4 Monte Carlo Method 42 4.0.3 MCNP5 . 43 4.0.4 Tallies within MCNP . 44 viii 4.0.5 Variance Reduction Methods . 46 5 Specialized Treatment for Non-Standard Vault Geometry 50 5.1 Vault Layout . 51 5.2 Analytic Model . 53 5.2.1 Maze Barrier Scatter . 54 5.2.2 Photon Scatter from the Maze Wall . 56 5.2.3 Total Scattered Photon Dose at the Vault Door . 57 5.3 Scattering Fractions . 58 5.3.1 Side Scatter Radiation . 58 5.3.2 Patient Scatter Fractions . 60 5.4 Simulation Geometry . 62 5.4.1 Materials and Source Definitions . 63 5.4.2 Tally Selection and Placement . 66 5.5 Results . 67 5.5.1 NCRP Formalism for Door Workload . 68 5.6 Recommendations for Additional Scattered Radiation Calculations . 70 6 Shielding Test Port Development 72 6.1 Shielding Test Port Development . 72 6.1.1 Risk Mitigation and Safety Interlocks . 73 6.1.2 Ease of Use and Block Design . 75 6.2 Parameters of Interest . 76 6.2.1 TVL Measurement . 76 6.2.2 Patient Scatter Fractions . 80 6.2.3 Differential Dose Albedo . 82 7 TVL Measurement and Simulation 84 ix 7.1 Experimental Setup . 84 7.1.1 TVL Measurement and Block Parameters . 84 7.1.1.1 Block Density Verification . 87 7.2 Monte Carlo Simulation . 88 7.2.1 Simulation Geometries . 88 7.2.2 Simulation Material and Source Specifications . 91 7.2.3 Variance Reduction and Tally Selection . 92 7.3 TVL Measurement and Simulation Results . 94 7.3.1 Measured TVL Thicknesses . 94 7.3.2 MCNP Simulation Results . 97 7.4 TVL Discussion . 100 8 Conclusions 110 References 112 A Fluence to Dose Conversion Factors from ICRP 116 126 x List of Tables 2.1 Dose Quality Factors for Different Types of Radiation . 10 2.2 Dose Weighting Factors for Different Body Tissues . 12 3.1 NCRP 151 Recommended TVLs . 26 3.2 NCRP 151 Recommended Occupancy Factors . 27 3.3 Patient-Scatter Fractions and TVLs . 31 3.4 NCRP 151 Concrete Differential Dose Albedos . 38 4.1 MCNP tallies . 44 5.1 Comparison of side scatter and patient scatter fractions . 60 5.2 Dimensions of the Truebeam vault . 62 5.3 Concrete compositions by weight . 65 5.4 Calculated maze scatter fractions compared to patient and side scatter fractions . 69 5.5 Photon Dose at the Door per Gy at Isocenter . 69 5.6 Total Photon Workload at Door per 1000 Gy . 70 7.1 Test block densities . 88 7.2 Measured vs. Published TVL1 for concrete . 95 7.3 Measured vs. Published TVL2 for concrete . 95 7.4 Measured TVLs 1, 2, & 3 . 96 7.5 Simulated TVL2 for concrete at various field sizes . 98 7.6 Simulated vs. NCRP recommended TVL1 for flattened beams . 99 xi 7.7 Simulated vs. NCRP recommended TVL2 for flattened beams . 99 7.8 Comparison of simulated TVL1 for flattened vs unflattened beams . 100 7.9 Comparison of simulated TVL2 for flattened vs unflattened beams . 100 7.10 Simulated TVLs out to TVL4 . 101 A.1 ICRP 116 photon fluence to effective dose conversion factors . 127 A.2 ICRP 116 neutron fluence to effective dose conversion factors . 128 xii List of Figures 1-1 Example of a standard vault with a maze . 2 1-2 Example of a vault with a direct shielded door .
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