Development of a methodology to study the effect of magnetic field on dose distributions in an MR- linac, using PRESAGE® and Monte Carlo calculations

At the Royal Marsden Hospital (RMH) and the Institute of Cancer Research (ICR) a new MR-linac is being installed and commissioned. Modifications to absorbed dose patterns will occur, because the paths of secondary electrons are deflected by Lorentz forces. In this paper, we describe a methodology to measure the effects of magnetic field on dose distributions, using the PRESAGE® 3D dosimeter and Monte Carlo (MC) simulations. A poly(methyl methacrylate) (PMMA) phantom has been developed to be positioned between the poles of an electromagnet and accommodate cylindrical samples of PRESAGE®. This phantom will be used to study the influence of different magnetic field strengths on radiation deposition from a Cobalt-60 (60Co) beam. Both the orientation of the samples with respect to the magnetic field and radiation beam, and the size of the air gap can be changed. Preliminary MC simulations with two PRESAGE® cylinders separated by an air gap, gave a good insight about the dosimetric effects that can be obtained with our newly-developed phantom.