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In magnetic resonance guided radiotherapy (MRgRT) radiation dose measurements needs to be performed in the presence of a magnetic field. In this study, the influence of magnetic fields on the readings of a Fricke detector, a chemical dosimeter, have been investigated in 6 MV photon beams. This type of detector has been chosen, as the Federal Office of Metrology (METAS, Switzerland) has great experience with Fricke dosimetry and since it is not expected that this detector is greatly affected by the presence of a magnetic field. Magnetic fields with field strengths between 0 T and 1.42 T were applied during the detector irradiation. In a 5 x 10 cmirradiation field, the Fricke readings are affected less than 0.9% by the applied magnetic fields. Taking the altered dose distribution due to the magnetic field (c) into account, the magnetic field correction factors (k) for the Fricke detector at 0.35 T and 1.42 T are determined to be 0.9948 and 0.9980, respectively. These small corrections hardly exceed the measurement uncertainties. Hence, we could proof that the Fricke detector is not significantly influenced by the presence of a magnetic field. The Fricke detector was also tested for the feasibility of measuring output factors in the presence of magnetic fields. For irradiation field sizes larger than the detector (> 2 x 2 cm), comparable results were obtained as for other detectors. The output factors decrease when a magnetic field is applied. This effect is more pronounce for larger magnetic field strengths and smaller irradiation fields due to shifts of the depth dose curves and asymmetry of lateral dose profiles.

physics in medicine and biology/physics in medicine and biology

Chemical radiation dosimetry in magnetic fields: characterization of a Fricke-type chemical detector in 6 MV photon beams and magnetic fields up to 1.42 T