Experimental characterisation of the magnetic field correction factor, k B →, for Roos chambers in a parallel MRI-linac

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Physics in Medicine and Biology


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. The kB→for the PTW Farmer chamber at 1 T on a parallel MRI-linac was 0.993 ± 0.013 (k = 1). The average kB→factor measured for the three Roos chambers on a 1 T parallel MRI-linac was 0.999 ± 0.014 (k = 1). Significance. The results presented are the first measurements of kB→for a Roos chamber on a parallel MRI-linac. The Roos chamber results demonstrate the potential for the chamber as a reference dosimeter in parallel MRI-linacs.

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National Health and Medical Research Council



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