Modelling the x-ray source for the Australian MRI-Linac

MRI-guided radiotherapy allows real-time imaging during treatment however the magnetic field influences the dose distribution in the patient. An accurate model of the radiation beam and the encompassing magnetic field is important to predict dosimetry changes. The purpose of this work is to develop a Monte Carlo model of the Australian MRI-Linac to be used as input into a dose calculation tool for treatment planning. The Australian MRI-Linac is a 1 T inline system with a 6MV flattening filter free photon beam. Commissioning measurements were undertaken both with and without the magnetic field present, PDDs and profiles were used to develop a model with the Geant4 toolkit. To date the model at 0 T matches within ± 2% of measured data. Ongoing work involves measurements at 1 T at various linac to MRI isocentre distances, the magnetic field model at each configuration is also under development.