Reference dosimetry for MRI-Linacs: an addendum to the 2020 IPEM code of practice for high-energy photon therapy dosimetry

The 2020 Institute of Physics and Engineering in Medicine code of practice (COP) for megavoltage dosimetry (<i>Eaton et al</i>2020<i>Phys. Med. Biol.</i><b>65</b>195006) provides standardised and practical methods for measurement of absorbed dose to water, linked to calibration by the National Physical Laboratory. This report extends the use of that COP to MRI-Linacs through the addition of two correction factors: (1) to account for the influence of the magnetic field on the absorbed dose and model-specific chamber response; and (2) to account for differences in chamber response in the beam spectrum of the MRI-Linac and the Linac where the cross-calibration is performed. Guidance is provided on practical aspects of chamber calibration and dose measurement in MRI-Linac beams, including the use of liquid water, consistent chamber orientation and corrections for other influence quantities. Using this approach uncertainties are similar to other applications of the COP.