Traceable reference dosimetry in MRI guided radiotherapy using alanine: calibration and magnetic field correction factors of ionisation chambers.

MRIgRT falls outside the scope of existing CoPs, because do not consider the effects of the magnetic field on detector response and on absorbed dose to water. The aim of this study is to evaluate and demonstrate the traceable measurement of absorbed dose in MRIgRT systems using alanine, made possible by the characterisation of alanine sensitivity to magnetic fields reported previously (Billas et al., 2020), in a way which is compatible with existing standards and calibrations available for conventional radiotherapy. In this study, alanine is used to transfer absorbed dose to water to MRIgRT systems from conventional linac. This alternative route for traceable measurement of absorbed dose to water is independent of the transfer using ionisation chambers. Alanine dosimetry is analysed in combination with measurements with several Farmer-type chambers, PTW/30013 and IBA/FC65-G, at six different centres and two different MRIgRT systems (Elekta Unity™ and ViewRay MRIdian™). The results are analysed in terms of the magnetic field correction factor, k QB,Q , and in terms of absorbed dose calibration coefficients for the chambers, determined at each centre. Results of following this approach to reference dosimetry in MRIgRT are consistent, across the centres visited, at the level of 0.41% (standard deviation). Farmer-type chamber k QB,Q determined directly, by comparing calibrations in some MRIgRT systems with and without the magnetic field ramped up, and indirectly, by comparing calibrations in all the MRIgRT systems with calibrations in a conventional linac. Calibration coefficients in the MRIgRT systems were obtained with a standard uncertainty of 1.06% (Elekta Unity™) and 0.92% (ViewRay MRIdian™), for three different chamber orientations. The values obtained for the k QB,Q in this investigation are consistent with those presented in the summary by de Pooter et al. (2020), and would tend to support the adoption of a k QB,Q which depends on the chamber type, PTW/30013 or IBA/FC65-G