Absolute Calibration of the Elekta Unity MR Linac Using the UK Code of Practice for High-Energy Photon Dosimetry

Absolute dosimetry for MR Linacs is complicated by non-standard reference conditions, the non-suitability of the UK secondary standard chamber for use in water and the effects of the magnetic field on the response of Farmer field chambers. Measurements were made on a pre-clinical 7 MV Elekta Unity MR Linac. Reference conditions were chosen as isocentre (143.5 cm SAD), 10 cm deep, 10 × 10 cm2 field and gantry angle 90° to avoid output variation from liquid helium levels dropping in the surrounding annulus. TPR 20/10 was measured as 0.698 at isocentre. Measurements on a conventional linac demonstrated that TPR 20/10 does not vary between SAD 100 cm and 143.5 cm. The UK National Physical Laboratory (NPL) provides calibration factors, ND, for the secondary standard NE 2611A thimble chamber in terms of absorbed dose to water. ND was taken for the value of the measured TPR 20/10. Independent intercomparisons were made at 6 MV on a conventional linac between the NE2611A and two PTW waterproof Farmer 30013 chambers. The response of these chambers varies very slowly with energy hence it is reasonable to assume this introduces minimal uncertainty. Ion recombination and polarity correction factors measured on the MR Linac were unchanged by the magnetic field. An additional correction factor to account for the 1.5 T magnetic field on the Farmer chambers was measured as 0.986 taking the ratio of TP corrected readings before and after ramp-up of the magnetic field. An orientation parallel to the magnetic field (along the bore of the MR Linac) was chosen to minimise the magnitude of this factor. An independent audit was performed by the NPL using alanine pellets and Farmer chambers calibrated via a water calorimeter from the Netherlands primary standards laboratory which operates within the MR Linac. This gave agreement with our calibration to within 1.0%.