Year

2020

Degree Name

Doctor of Philosophy

Department

School of Physics

Abstract

External beam radiation therapy (EBRT) is one of the most common forms of therapy used to treat cancers worldwide. New EBRT modalities are being developed across a range of therapies to improve patient outcomes. With these new therapies comes unique challenges for dosimetry and quality assurance which is vital for safe and effective treatments. Microbeam radiation therapy (MRT) is a novel form of EBRT that uses spatially fractionated synchrotron sourced kilovoltage x-rays to deliver highly-targeted planes of x-rays to the target volume. These microbeams are typically 25 – 50 µm in thickness, with a peak-to-peak separation of 200 – 400 µm . MRT is currently limited to synchrotron sources, and is therefore only being studied at a small number of facilities worldwide, but the emergence of compact synchrotron technology has the potential to make MRT much more accessible. Due to the extreme dose rates of MRT (in excess of 5000 Gy/s in some configurations) and high spatial fractionation of the microbeams, MRT dosimetry is an ongoing challenge that must be solved if it is to be used in human clinical trials.

In this thesis scintillator fibre-optic dosimeters (FODs) with the highest spatial res- olution found in the literature are developed and tested at the Australian Synchrotron on the Imaging and Medical BeamLine, where MRT cell and animal studies are being conducted. These scintillator FODs use plastic scintillator due to its water-equivalence, energy and temperature independence and radiation hardness. The scintillator is fab- ricated to have a thickness equal to the desired one-dimensional spatial resolution for microbeam dosimetry. In this thesis scintillator probes with thicknesses 50 µm , 20 µm , and 10 µm are presented. The scintillator is coupled to an optical fibre with core diameter 1.0 mm. When oriented with the thin edge of the scintillator towards the beam, a microbeam array can be scanned with this high resolution. The FODs were able to resolve microbeams with 50 µm full-width at half-maximum, with the 20µm and 10 µm FODs showing the best agreement to expected values. Only the 10 µm FOD was able to accurately measure the dose in the valleys between the microbeams. Simulations of the detector and the synchrotron beamline were done using the physics particle simulation toolkit Geant4. These simulations were able to provide predictions of the expected intrinsic dose distributions in the microbeam fields, as well as what signals to expect from the FODs. These provided an estimate of the ideal detector responses for the experimental results to be evaluated against. These simulations showed that both the 20 µm and 10 µm detectors measured the full- width at half-maximum that was expected, while the 50 µm detector performed less accurately than could be expected.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.