Background: The static magnetic field present in magnetic resonance (MR)-guided radiotherapy systems can influence dose deposition and charged particle collection in air-filled ionization chambers. Thus, accurately quantifying the effect of the magnetic field on ionization chamber response is critical for output calibration. Formalisms for reference dosimetry in a magnetic field have been proposed, whereby a magnetic field quality conversion factor k(B,Q) is defined to account for the combined effects of the magnetic field on the radiation detector. Determination of k(B,Q) in the literature has focused on Monte Carlo simulation studies, with experimental validation limited to only a few ionization chamber models.Purpose: The purpose of this study is to experimentally measure k(B,Q) for 11 ionization chamber models in two commercially available MR-guided radiotherapy systems: Elekta Unity and ViewRay MRIdian.Methods: Eleven ionization chamber models were characterized in this study: Exradin A12, A12S, A28, and A26, PTW T31010, T31021, and T31022, and IBA FC23-C, CC25, CC13, and CC08. The experimental method to measure k(B,Q) utilized cross-calibration against a reference Exradin A1SL chamber. Absorbed dose to water was measured for the reference A1SL chamber positioned parallel to the magnetic field with its centroid placed at the machine isocenter at a depth of 10 cm in water for a 10 x 10 cm(2) field size at that depth. Output was subsequently measured with the test chamber at the same point of measurement. k(B,Q) for the test chamber was computed as the ratio of reference dose to test chamber output, with this procedure repeated for each chamber in each MR-guided radiotherapy system. For the high-field 1.5 T Elekta Unity system, the dependence of k(B,Q) on the chamber orientation relative to the magnetic field was quantified by rotating the chamber about the machine isocenter.Results: Measured k(B,Q) values for our test dataset of ionization chamber models ranged from 0.991 to 1.002, and 0.995 to 1.004 for the Elekta Unity and ViewRay MRIdian, respectively, with k(B,Q) tending to increase as the chamber sensitive volume increased. Measured k(B,Q) values largely agreed within uncertainty to published Monte Carlo simulation data and available experimental data. k(B,Q) deviation from unity was minimized for ionization chamber orientation parallel or antiparallel to the magnetic field, with increased deviations observed at perpendicular orientations. Overall (k = 1) uncertainty in the experimental determination of the magnetic field quality conversion factor, k(B,Q) was 0.71% and 0.72% for the Elekta Unity and ViewRay MRIdian systems, respectively.Conclusions: For a high-field MR-linac, the characterization of ionization chamber performance as angular orientation varied relative to the magnetic field confirmed that the ideal orientation for output calibration is parallel. For most of these chamber models, this study represents the first experimental characterization of chamber performance in clinical MR-linac beams. This is a critical step toward accurate output calibration for MR-guided radiotherapy systems and the measured k(B,Q) values will be an important reference data source for forthcoming MR-linac reference dosimetry protocols.
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