UNIVERSITÀ DEGLI STUDI DI CAGLIARI FACOLTÀ DI SCIENZE MATEMATICHE, FISICHE E NATURALI Scuola di Dottorato in Scienze e Tecnologie Fisiche XX Ciclo Monte Carlo simulation of dose distributions for synchrotron Microbeam Radiation Therapy Ph. D. Thesis by Jenny Spiga Supervisors: Prof. Paolo Randaccio Dott. Alberto Bravin Contents List of Figures v List of Tables ix Introduction 1 Chapter 1. Principles and practice of radiation therapy 1.1 Introduction ............................................................................................................................... 6 1.2 Effects of radiation on biological tissues................................................................................... 7 1.3 Different types of radiation therapy .......................................................................................... 8 1.3.1 Modern technological radiotherapy techniques .................................................................... 9 1.4 Limitations of radiotherapy for childhood brain tumors ......................................................... 11 1.5 Synchrotron X-ray microbeam radiation therapy (MRT) ........................................................ 11 1.5.1 Effectiveness of spatially fractionated radiotherapy ........................................................... 11 1.5.2 MRT preclinical research programs ................................................................................... 13 1.5.3 Determination of dose distributions using MC calculations ............................................... 15 1.5.4 Experimental dosimetry ..................................................................................................... 17 1.6 Conclusions and thesis motivations ........................................................................................ 17 Chapter 2. Synchrotron radiation for MRT 2.1 Introduction to synchrotron radiation ...................................................................................... 22 2.2 Synchrotron radiation sources ................................................................................................. 23 2.3 Beam characteristics ................................................................................................................ 27 2.3.1 Brilliance ............................................................................................................................ 27 2.3.2 Divergence ......................................................................................................................... 27 2.3.3 Polarization ........................................................................................................................ 28 2.4 The European Synchrotron Radiation Facility (ESRF) ........................................................... 28 2.5 The beamlines ......................................................................................................................... 29 2.5.1 The ID17 bio-medical beamline ......................................................................................... 30 2.5.2 The experimental station for MRT ..................................................................................... 32 Chapter 3. Fundamentals of the GEANT4 Monte Carlo code 3.1 Monte Carlo techniques in Radiation Transport ..................................................................... 38 3.1.1 Photon interaction processes .............................................................................................. 38 3.1.2 Photon cross sections and transport algorithm ................................................................... 39 3.1.3 Electron interaction processes ............................................................................................ 40 3.1.4 Multiple scattering theories ................................................................................................ 41 3.1.5 Condensed History technique ............................................................................................. 42 3.1.6 Electron transport algorithm ............................................................................................... 43 3.2 Monte Carlo codes for radiotherapy applications ................................................................... 43 3.3 GEANT4 structure .................................................................................................................. 44 3.3.1 Mandatory user classes ....................................................................................................... 44 3.4 Geometry construction ............................................................................................................ 46 3.4.1 Volume definitions ............................................................................................................. 46 3.4.2 Repeated volumes .............................................................................................................. 46 i Contents p 3.5 Particle description .................................................................................................................. 46 3.6 Physics processes .................................................................................................................... 47 3.6.1 Determination of step size and particle interaction type .................................................... 48 3.6.2 Particle tracking algorithm ................................................................................................. 49 3.7 Scoring techniques .................................................................................................................. 50 3.8 Visualization tools ................................................................................................................... 50 Chapter 4. Simulation results: cylindrical geometry 4.1 Introduction ............................................................................................................................. 54 4.2 Simulation geometry and details ............................................................................................. 54 4.3 Lateral dose profiles ................................................................................................................ 56 4.3.1 Dose deposition for different microbeam energies ............................................................. 56 4.3.2 Study of the effect of the microbeam’s width .................................................................... 58 4.4 Depth dose profiles ................................................................................................................. 59 4.5 Peak to valley dose ratios ........................................................................................................ 61 4.5.1 Factors that influence PVDRs ............................................................................................ 63 4.6 The impact of different physical processes ............................................................................. 64 Chapter 5. Simulation results: planar geometry 5.1 Introduction ............................................................................................................................. 70 5.2 Simulation geometry and details ............................................................................................. 70 5.3 Lateral dose profiles ................................................................................................................ 71 5.3.1 Dose deposition for different microbeam energies ............................................................. 71 5.3.2 Study of the effect of the microbeam’s width .................................................................... 73 5.3.3 Dose deposition for different microbeam shapes ............................................................... 74 5.3.4 Study of the effect of the phantom material ....................................................................... 75 5.4 Peak to valley dose ratios ........................................................................................................ 76 5.4.1 Determinants of PVDRs ..................................................................................................... 79 Chapter 6. Simulation results: comparison of different MC codes 6.1 Introduction ............................................................................................................................. 84 6.2 Brief description of the codes .................................................................................................. 85 6.2.1 The PENELOPE code ........................................................................................................ 85 6.2.2 The EGS code..................................................................................................................... 86 6.2.3 The MCNPX code .............................................................................................................. 87 6.3 Simulation geometry and details ............................................................................................. 90 6.4 GEANT4-LowE versus GEANT4-PENELOPE ..................................................................... 90 6.5 GEANT4 versus other Monte Carlo codes .............................................................................. 92 6.5.1 Dose calculations using cylindrical beams ......................................................................... 93 6.5.2 Dose calculations