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Magnetic resonance (MR) imaging systems in radiotherapy offer the possibility of acquiring high quality images with high soft-tissue contrast without adding extra dose to the patient. The MR system combined with a linear accelerator allows for on-table adaptive radiotherapy and also gated treatment using live images as the patient is being treated. This in turn may allow for margin reductions and dose escalation. However, the implementation increases the challenges for the medical physicist as the dosimetry under magnetic fields has not been incorporated in the currently used radiotherapy protocols (TRS-398, TG-51, TRS-483). The objective of this study is to investigate the performance of the ionization chamber PTW-31021 Semiflex 3D under a 0.35 tesla magnetic field for reference dosimetry using the magnetic field correction factor.

The magnetic field correction factor (k_B) was calculated using Monte Carlo simulations. The Monte Carlo based toolkit EGSnrc was used for the computations. The radiation source used was a 6 MV flattening filter free beam as this study aims to incorporate experimental measurements using a ViewRay MRIdian accelerator. The impact of the magnetic field was evaluated with the ionization chamber positioned perpendicular to the beam direction, and in parallel and perpendicular orientation with respect to the magnetic field. In addition, the ionization chamber was positioned parallel to beam orientation and parallel to the magnetic field.  In order to compare to an often used ionization chamber for reference dosimetry the PTW-30013 Farmer was also simulated. This ionization chamber cannot be used parallel to the beam direction therefore the k_B factor was computed for this detector merely perpendicular to the beam and with an orientation parallel and perpendicular to the magnetic field.

The magnetic field correction factor for the Farmer ionization chamber agrees with those reported in the literature.  The correction is approximately 0.3% in the parallel orientation and ~3 % for the perpendicular orientation. For the Semiflex (perpendicular to the beam) the corrections are higher than for the Farmer, yielding a correction of ~1.5% for the parallel orientation and ~3.5% for the perpendicular one. However, when using the Semiflex ionization chamber in a parallel orientation with respect the beam direction the correction is ~0.2%.

The performance of the Semiflex 3D ionization chamber for reference dosimetry purposes under a magnetic field of 0.35 tesla, was assessed. The results suggest that the Semiflex 3D ionization chamber has a better behavior than the Farmer chamber, for reference dosimetry, if used in a parallel orientation with respect the beam direction. Further investigation of the perturbation factors under magnetic fields for this detector can add a better understanding of the problem.