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Objective. Reference dosimetry on an MRI-linac requires a chamber specific magnetic field correction factor, kB -. This work aims to measure the correction factor for a parallel plate chamber on a parallel MRI-linac. Approach. kB→is defined as the ratio of the absorbed dose to water calibration coefficient in the presence of the magnetic field, ND,wB→relative to that under 0 T conditions, ND,w0T. kB→was measured via a ND,w transfer to a field chamber at each magnetic field strength from a chamber with known ND,w and kB -. This was achieved on the parallel MRI-linac by moving the measurement set-up between a high magnetic field strength region at the MRI-isocentre and a low magnetic field strength region at the end of the bore whilst maintaining consistent set-up and scatter conditions. Three PTW 34001 Roos chambers were investigated as well as a PTW 30013 Farmer used to validate methodology. Main Results. The beam quality used for the measurements of kB→was TPR 20/10 = 0.632. T

Absorbed doseMagnetic fieldRi linacsOos chamber