The Monte Carlo method is an integral part of existing radiotherapy reference dosimetry. This technique not only allows the characterisation of the physical properties of beams and radiation detectors with high accuracy, it also serves as a reference method for a variety of situations such as providing valuable data for clinical dosimetry. In the context of MR guided radiotherapy (MRgRT), recent studies have shown significant effects on absorbed dose in the presence of magnetic fields (B-fields). Monte Carlo is expected to play a key role in calibrating MRgRT machines.

The aim of this work package is to develop methodologies for testing and validating Monte Carlo based radiation transport algorithms for accurate beam modelling and detector response simulations in external magnetic fields. General-purpose codes integrating B-fields are yet to be properly tested in calculations relevant to reference dosimetry for both self-consistency and experimental benchmarks. The benchmarked Monte Carlo codes for application in magnetic fields will then be used to model MRgRT facilities using a method previously applied for conventional radiotherapy facilities. The results of these beam simulations will be used as input for WP2. Furthermore, the benchmarked Monte Carlo codes will be used to calculate correction factors (kB) for a set of reference ionisation chambers and other detectors, which will be used as input for WP1 and WP2.