MR‐guided proton therapy: Impact of magnetic fields on the detector response

PURPOSE To investigate the response of detectors for proton dosimetry in the presence of magnetic fields. MATERIAL & METHODS Four ionization chambers, two thimble-type and two plane-parallel-type, and a diamond detector were investigated. All detectors were irradiated with homogeneous single-energy-layer fields, using 252.7 MeV proton beams. A Farmer ionization chamber was additionally irradiated in the same geometrical configuration, but with a lower nominal energy of 97.4 MeV. The beams were subjected to magnetic field strengths of 0, 0.25, 0.5, 0.75 and 1 T produced by a research dipole magnet placed at the room's isocenter. Detectors were positioned at 2 cm water-equivalent depth, with their stem perpendicular to both the magnetic field lines and the proton beam's central axis, in the direction of the Lorentz force. Normality and two sample statistical Student's t-tests were performed to assess the influence of the magnetic field on the detectors' responses. RESULTS For all detectors, a small but significant magnetic-field-dependent change of their response was found. Observed differences compared to the no magnetic field case ranged from +0.5% to -0.7%. The magnetic field dependence was found to be non-linear and highest between 0.25 and 0.5 T for 252.7 MeV proton beams. A different variation of the Farmer chamber response with magnetic field strength was observed for irradiations using lower energy (97.4 MeV) protons. The largest magnetic field effects were observed for plane-parallel ionization chambers. CONCLUSION Small magnetic-field-dependent changes in the detector response were identified, which should be corrected for dosimetric applications.