On the characterisation of stereotactic radiotherapy fields

Cancer is one of the leading causes of deaths in Australia, with a mortality rate of approximately 40,000 deaths per year, contributing $3.8 billion AUD in direct health system costs. One advanced treatment modality for small tumours is stereotactic radiotherapy, which employs multiple beams of ionising radiation that spatially conform to a targeted lesion, using higher radiation doses in fewer fractions compared to other methods. This is increasingly popular because of patient convenience and an expectation of higher cure rates.<br><br>This work investigates and characterises stereotactic radiotherapy fields with the objective of improved treatments and hence better patient outcomes.<br><br>Calculation and measurement of in-field characteristics is complicated by issues such as electronic disequilibrium, spectral changes and detector volume averaging effects (when the detector is of comparable or larger size than the radiation field). In this work, 3D dosimetric methods based on radiosensitive gels are developed and implemented for dose measurement, and sophisticated mathematical Monte Carlo radiation transport models are constructed and applied for accurate beam characterisation.<br><br>Out-of-field doses (i.e. beyond the targeted region) are of interest for the potential health complications they may give rise to, such as radiocarcinogenesis, cardiac and respiratory problems. Comparatively little attention is given to out-of-field doses from stereotactic fields, which this study investigates both systematically and in the context of paediatric radiotherapy, providing risk estimates for radiation-induced cancer.<br><br>Key findings relate to the radiological properties and calibration of 3D gel dosimeters. Monte Carlo models reveal the spectral characteristics of stereotactic fields within and beyond the nominal treatment field, and these are investigated in terms of the effect on energy-dependent dosimeters. Investigations of out-of-field dose have revealed anisotropies in the radiation field far from the primary beam which may be exploited so as to significantly minimise patient dose and corresponding health risks.<br><br>The present work has yielded 11 publications in international peer-reviewed journals, a further 3 publications currently under review or preparation, 19 conference papers and 7 invited seminars.