Speaker
Description
FLASH radiotherapy (RT) is an emerging cancer treatment modality that utilises much higher dose rates than conventional RT. Delivering radiation with ultra-high dose rates (UHDR) has been shown to spare healthy tissue while providing an equal or greater dose to the tumour. Current research suggests the use of very high-energy electrons can provide further benefits, such as treatment of deep seated tumours. Linacs have been used to deliver ultra-high dose rate electrons, with current dosimetry utilising radiochromic film. However, film does not provide the real-time results required in a clinical setting. The MOSkin detector, designed at the Centre For Medical Radiation Physics at University of Wollongong is used in conventional radiotherapy and considered dose-rate independent over a limited range. This study aims to show that dose-rate independence continues to exist when exposed to a UHDR, very high energy electron (VHEE) beam.
The Australian Synchrotron uses a linac to inject 100 MeV electrons, capable of delivering pulses with expected dose rates of $10^7$ Gy/s. The linac lacks beam scanning or positioning equipment so an array of five detectors was designed, built and manually positioned with the assistance of a portable laser. An x-ray intensifier screen was positioned behind the array and imaged with a camera, to collect spatial data and relative beam intensity between pulses. 13 beam currents were used to deliver 300 pC pulses from 20 ns to 400 ns in length.
The detector with the highest response was assumed to be closest to the beam centre and an average response for each beam current was calculated. Dose rates for each pulse were estimated using a standard MOSkin calibration factor and range from approximately $7x10^5$ – $2.5x10^7$ Gy/s. A steep drop off in response was observed at beam currents below 2 mA. Beam profiles were created using the camera data, with a Moffat distribution fitted to determine relative intensity between pulses at the detector's estimated location. The amplitude was extracted from the distributions and normalised to 1, which enabled plotting against normalised MOSkin data to evaluate detector response against the charge delivered. The MOSkin response is consistent with the x-ray intensifier screen and indicates pulses are being delivered to the experimental stage at lower beam current than that measured by the linac diagnostics.
While the imaging equipment cannot provide an estimate of dose, it explains the variance in detector behaviour between pulses, especially at lower dose rates. While uncertainty is large due to manual positioning, the experiment has shown that further investigation is justified as the MOSkin appears suitable for dosimetry in UHDR VHEE FLASH environments. Future experiments will be conducted to gain better spatial information as well as an independent measurement of dose.
