Year

2021

Degree Name

Doctor of Philosophy

Department

School of Physics

Abstract

The integration of online magnetic resonance imaging (MRI) with photon and pro-ton radiotherapy has potential to overcome the soft tissue contrast limitations of the current standard of care kV-image guided radiotherapy in some challenging treat-ment sites. By directly visualising soft tissue targets and organs at risk, removing the dependence on surrogates for image guidance, it is expected there will be a decrease in the geometric uncertainties related to daily patient setup. This new approach to image guided radiotherapy presents unique challenges due to the permanent mag-netic field of the integrated MRI unit. The trajectory of charged particles including dose depositing secondary electrons are perturbed by the magnetic field, adding to the challenge of calculating the patient dosimetry and validating the calculation with measurement as is standard practice in radiotherapy. The magnetic field may also effect the operation and response of radiation detectors and a method of accurately characterising the influence of the magnetic field on detector response and operation is required.

This thesis reports progress made towards real time high spatial resolution dosime-try of photon and proton MRI guided radiotherapy beams using novel monolithic silicon detectors designed at the Centre for Medical Radiation Physics (CMRP). One challenge in experimentally characterising the magnetic field effects on a radiation detectors operation is how to perform dosimetry measurements with and without a magnetic field of varying strength and orientation from a single radiation source as this is not feasible on existing MRI linacs with a permanent magnetic field of fixed strength. A bespoke semi-portable magnet device was developed to meet this need. The device employs an adjustable iron yoke and focusing cones to vary the magnetic field of the central volume, a 0.3 T field can be achieved for volume to 10 x 10 x 10 cm3 and up to a 1.2 T for a volume of at least 3 x 3 x 3 cm3. The device is de-signed to be used with a clinical linear accelerator in both inline and perpendicular magnetic field orientations to meet the challenge of detector characterisation. The performance of the magnetic field generated by the device was within ±2 % of finite element modelling predictions of all configurations tested.

FoR codes (2008)

029903 Medical Physics

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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.