My PhD Story

I thank

• Cancer Research Wales and Yr Ysgol Uwchradd Tregaron, for funding.
• Geraint Lewis and Cyril Smith, for the incredible opportunities.
• Emiliano Spezi, for the tradition of excellence.
• Jeffrey Siebers, for co-supervising part of the work (Chapter 6).
• Paul Keall et al. at VCU and Phil Evans et al. at ICR/RMH, for imaging facilities and helpful discussion.
• Frank Verhaegen and Amanda Barry, for reviewing the thesis.
• Rebecca Cufflin, for proof-reading the thesis.
• Jonathan Giddy, for facilitating High Throughput Computing services.
• colleagues at Velindre Cancer Centre, for friendship and tremendous support.
• Richard Jarvis, for assistance with image acquisition.
• Mel Jones et al., for making the phantoms.
• Andrew Edwards, for fixing the bike.
• colleagues in the scientific community far and near, for encouragement.
• housekeepers and porters at Neuadd Meirionnydd, for laughter.
• Sr Hermine, for being an inspiration still.
• Dato Dr Sivananthan and the late Dato Dr Pathmanathan, for medical care when I was tiny.
• physics, for 16 years of fascination.
• Brecon Beacons, for your contours of beauty.
• and the ponies, for the rhythm.
• friends and family 7000 miles away, for blessings.
• Sayang, for sayang.

Title

Monte Carlo portal dosimetry

Abstract

This project developed a solution for verifying external photon beam radiotherapy. The solution is based on a calibration chain for deriving portal dose maps from acquired portal images, and a calculation framework for predicting portal dose maps. Quantitative comparison between acquired and predicted portal dose maps accomplishes both geometric (patient positioning with respect to the beam) and dosimetric (2D fluence distribution of the beam) verifications. A disagreement would indicate that beam delivery had not been according to plan. The solution addresses the clinical need for verifying radiotherapy both pre-treatment (without patient in the beam) and on-treatment (with patient in the beam). Medical linear accelerators mounted with electronic portal imaging devices (EPIDs) were used to acquire portal images. Two types of EPIDs were investigated: the amorphous silicon (a-Si) and the scanning liquid ion chamber (SLIC). The EGSnrc family of Monte Carlo codes were used to predict portal dose maps by computer simulation of radiation transport in the beam-phantom-EPID configuration. Monte Carlo simulations have been implemented on several levels of High Throughput Computing (HTC), including the Grid, to reduce computation time. The solution has been tested across the entire clinical range of gantry angle, beam size (5 cm x 5 cm to 20 cm x 20 cm), beam-patient and patient-EPID separations (4 cm to 38 cm). In these tests of known beam-phantom-EPID configurations, agreement between acquired and predicted portal dose profiles was consistently within 2% of the central axis value. This Monte Carlo portal dosimetry solution therefore achieved combined versatility, accuracy and speed not readily achievable by other techniques.

Table of contents

1. Chapter 1. Introduction
   • Radiotherapy
   • Imaging during radiotherapy
   • Monte Carlo simulations
   • Grid computing
   • Objective & context
   • Thesis structure
2. Chapter 2. Electronic Portal Imaging
   • Imaging performance
   • Megavoltage imaging
   • The SLIC and the a-Si technologies
3. Chapter 3. Monte Carlo Radiation Transport
   • Analog and non-analog simulations
   • Why Monte Carlo
   • Choice of codes
   • EGSnrc, BEAMnrc and DOSXYZnrc
4. Chapter 4. Simulation of Linac Sources
   • Introduction
   • Materials & methods
     • Migration from BEAM00 to BEAMnrc
     • Selection of transport options
     • Fine-tuning the linac model
     • Modelling varying field sizes
     • Use of phase space particles
   • Results & discussion
   • Conclusions
5. Chapter 5. A Monte Carlo Solution
   • Introduction
   • Materials & methods
     • Calibrations
       • Image calibration
       • Grayscale-to-dose calibration
       • Off-axis calibration
     • Phantom packing using TWIZ&GLU
     • Demonstrations
   • Results & discussion
   • Conclusions
6. Chapter 6. Optimisation of Monte Carlo Simulations
   • Introduction
   • Materials & methods
     • Radiation transport studies
     • Design of simplified EPID models
       • a simplified a-Si EPID model
       • a simplified SLIC EPID model
     • Selection of transport options
     • Comparison with other techniques
   • Results & discussion
     • Radiation transport studies
       • The a-Si
       • The SLIC
     • Conclusions
   • Chapter 7. Practical Aspects of Image Acquisition
     • Introduction
     • Materials & methods
       • The SLIC: variation with gantry angles
         • Angular dependence of images
         • Backscatter from surroundings
         • Reproducibility
         • Angular correction function
         • Comparison with existing techniques
         • Adaptation of the new technique
       • The a-Si: an artefact in the IMRT acquisition mode
       • The a-Si: variation with gantry angles
       • Ideas for IMRT verification
         • An alternative imaging sequence
         • Rethinking ghosts
     • Results & discussion
     • Conclusions
   • Chapter 8. High Throughput Computing
     • Introduction
     • Materials & methods
       • Four levels of implementation
       • Simulations on multiple platforms
       • Planning of simulations
       • Simulations without pre-installation
     • Results & discussion
     • Conclusions
   • Chapter 9. Conclusions
   • Appendix A. Dissemination of work

Full dissertation

Slideshows

An intro to the lay(wo)man (191 KB .pdf)

Highlights from my PhD thesis: as presented during viva (623 KB .pdf)

My PhD story: as presented at the Royal Marsden / Institute of Cancer Research Nov 200 (.pdf)