Radiaton Dosimetry of Conventional and Laser-Driven Particle Beams
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RADIATION DOSIMETRY OF CONVENTIONAL AND LASER-DRIVEN PARTICLE BEAMS by DANIEL KIRBY A thesis submitted to The University of Birmingham for the degree of DOCTOR OF PHILOSOPHY School of Physics and Astronomy College of Engineering and Physical Sciences The University of Birmingham April 2011 University of Birmingham Research Archive e-theses repository This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder. Abstract The measurement of radiation dose in radiotherapy is vital in ensuring the accuracy of treatments. As more advanced techniques using protons and ions emerge, they pose chal- lenges to ensure the same level of accuracy of dosimetry is achieved as for conventional X-ray radiotherapy. A relatively new method of particle acceleration using ultra-high intensity lasers and thin metallic targets has sparked a large effort to investigate the possible application of this technology in radiotherapy, which in turn requires accurate methods of dosimetry to be carried out and is the main motivation for this work. Ac- curate dosimetry was initially performed here using an air ionisation chamber, various models of GafChromic film and a PMMA phantom in 15 and 29 MeV protons and 38 MeV α-particles from the Birmingham cyclotron. In developing an accurate protocol for absorbed dose-to-water at these relatively low proton energies, new data was generated on the proton energy response of GafChromic films. This enabled accurate dosimetry of a prototype laser-particle source, and provided improvements to a method of spectroscopic measurement in the resultant mixed field of multi-energy protons, electrons and X-rays. Monte Carlo simulations using MCNPX but mainly FLUKA were performed throughout to support and verify experimental measurements. This thesis is dedicated to my ever supportive wife Heidi, and baby daughter Eva Grace for making my life richer beyond imagination. ACKNOWLEDGEMENTS Seldom is any real work achieved without the significant contribution of others. At the top of the list is my supervisor Dr. Stuart Green for his hard work and dedication in supervising not only myself but all of his students. His enthusiasm in the development of novel radiotherapy technologies has been a true inspiration. The vast majority of my work was performed using the Birmingham cyclotron, run by Prof. David Parker and his team Mike Smith, Greg Wood and John Lowe who showed great patience and extreme helpfulness for which I am eternally grateful. I would also like to thank Dr. Hugo Palmans for sharing his great knowledge of particle dosimetry and providing further support whenever I needed it and my fellow student and friend Francesca Fiorini for all her help, especially with FLUKA, and keeping me company on many trips to Belfast and other meetings and conferences. Others who have played a significant role in my work and deserve many thanks are Cecile Wojnecki, Richard Hugtenburg, Marco Borghesi and the group at Queens Univer- sity Belast, and many people at International Specialty Products Corp. for going to great lengths to support my research with GafChromic film. I also wish to thank the School of Physics and Astronomy for providing my doctoral stipend. Lastly, I want to thank my wonderful wife Heidi and all my family for their support during not only my doctoral studies but also for a patience-testing total of nine years at University. CONTENTS 1 Introduction to particle energy loss, acceleration and radiotherapy1 1.1 Principles of ion beam therapy.........................1 1.1.1 Interactions of heavy charged particles................1 1.1.2 Dose distributions of protons and ions................8 1.1.3 Radiobiological benefit of ions..................... 10 1.1.4 Treatment delivery techniques..................... 13 1.2 Methods of ion acceleration.......................... 15 1.2.1 Cyclotron................................ 15 1.2.2 Synchrotron............................... 17 1.2.3 Promising alternatives......................... 18 1.3 Laser-plasma acceleration and LIBRA.................... 19 1.3.1 Introduction and motivation...................... 19 1.3.2 Overview of theory........................... 23 1.4 Scope of work.................................. 27 2 Methods of ion dosimetry 28 2.1 Code of practice for proton dosimetry..................... 28 2.2 Ionisation chamber dosimetry......................... 31 2.2.1 Cavity theory.............................. 34 2.2.2 Absorbed dose-to-water formalism................... 37 2.2.3 Beam quality correction........................ 38 2.2.4 Ion recombination............................ 39 2.2.5 Temperature and pressure correction................. 42 2.3 Radiochromic film................................ 42 2.3.1 Overview of GafChromic film..................... 44 2.3.2 Energy dependence........................... 48 2.3.3 Dose-rate dependence.......................... 50 2.3.4 Temperature effects........................... 51 2.3.5 Non-uniformity issues.......................... 52 2.3.6 Light and polarisation effects..................... 54 3 Monte Carlo radiation transport methods 57 3.1 Monte Carlo charged particle transport.................... 58 3.2 The MCNPX code............................... 59 3.3 The FLUKA code................................ 61 3.4 The SRIM program............................... 62 3.5 Parallel simulations with BlueBEAR..................... 63 3.6 Code comparison................................ 64 4 Ionisation chamber dosimetry 68 4.1 Dosimetry equipment and materials...................... 68 4.1.1 The Perspex dosimetry jig and beam port.............. 68 4.1.2 Ionisation chambers........................... 70 4.2 Dosimetry of 15 and 29 MeV protons..................... 71 4.2.1 Experimental method.......................... 72 4.2.2 MCNPX and FLUKA simulations................... 75 4.2.3 Recombination (kion) correction.................... 77 4.2.4 Beam quality (kQ) correction..................... 81 4.2.5 Fully corrected depth-dose measurements............... 84 4.3 Dosimetry of 38 MeV α-particles....................... 86 4.3.1 Experimental method.......................... 86 4.3.2 FLUKA simulations.......................... 87 4.3.3 Beam quality (kQ) correction..................... 87 4.3.4 Recombination (kion) correction.................... 88 4.3.5 Fully corrected depth-dose measurements............... 91 5 GafChromic film dosimetry 93 5.1 Nikon Super Coolscan 35mm film scanner.................. 93 5.1.1 Specifications.............................. 94 5.1.2 Operational tests............................ 96 5.2 Concept of gQ;Q0 beam quality correction factor............... 103 5.2.1 Definition of gQ;Q0 ........................... 103 5.2.2 Film stopping powers.......................... 104 5.3 Dosimetry of 15 and 29 MeV protons..................... 110 5.3.1 Calibration with 6 MV X-rays..................... 110 5.3.2 Depth-dose measurements....................... 112 5.3.3 Relative effectiveness.......................... 114 5.4 FLUKA simulation of GafChromic film proton response........... 121 5.5 Dosimetry of 38 MeV α-particles....................... 121 5.5.1 Calibration............................... 123 5.5.2 Depth-dose measurements....................... 125 5.5.3 Relative effectiveness.......................... 126 6 Laser-accelerated proton dosimetry and spectroscopy 131 6.1 Dosimetry and RCF stack spectroscopy of a laser-proton source...... 132 6.1.1 Introduction............................... 132 6.1.2 GafChromic film calibration...................... 133 6.1.3 Post-exposure OD growth....................... 135 6.1.4 Main experimental method....................... 137 6.1.5 Monte Carlo method.......................... 138 6.1.6 Spectrum search algorithm....................... 141 6.1.7 Results.................................. 142 6.2 Towards cell survival measurements in a laser-proton source........ 146 6.2.1 Introduction............................... 146 6.2.2 Overview of setup............................ 147 6.2.3 FLUKA simulations.......................... 150 6.2.4 Proposed dosimetry method...................... 155 6.2.5 EBT2 response verification....................... 156 7 Summary 161 7.1 Ionisation chamber dosimetry......................... 161 7.2 Relative effectiveness of GafChromic film................... 162 7.3 GafChromic film spectroscopy of a laser-proton source............ 163 7.4 Laser-proton radiobiology experiment..................... 164 7.5 Miscellaneous work............................... 164 7.5.1 Other LIBRA collaborative work................... 164 7.5.2 FLUKA and CT data import..................... 166 7.5.3 Half-range modulator wheel for 29 MeV protons........... 167 7.6 Future work................................... 169 References 171 Appendix A: Full GafChromic film composition data 182 Appendix B: Fortran code 183 B.1 FLUBOUND program............................. 184 B.2 RCF spectrum solver.............................. 187 Appendix C: BASH scripts 192 C.1 BlueBEAR submission example (FLUKA).................. 192 C.2 Combining FLUKA output files........................ 194