Ionisation chamber readings for proton beams in magnetic fields require dose correction factors

Objectives: Proton therapy (PT) reduces the integral dose in healthy tissue compared to X-ray therapy (XT). At the same time PT is more sensitive to anatomical changes and organ motion than XT, making MRI guidance especially relevant for PT. Recently, we have integrated for the first time a 0.22 T in-beam MRI scanner with a proton therapy research beamline. As a next step, absolute and relative dosimetry in the presence of magnetic fields of the MRI scanner has to be established. This work investigates the influence of the static magnetic (B0) field of the MRI scanner on ionisation chamber (IC) readings for proton beams and the feasibility to correct the readings for the B0 field.

Materials & methods: Either a Semiflex 0.3 or a PinPoint3D IC was positioned in an MR-compatible water phantom to measure the absolute dose at five proton energies (70, 110, 150, 190, 226.7 MeV) in the entrance plateau area of the Bragg curve for a 10 x 10 cm2 pencil beam scanned irradiation field. The entrance window of the phantom was placed at the imaging isocentre of the MRI scanner. Dose measurements were repeated at the same position and under the same conditions without the B0 field. The IC signal was corrected to standard environmental conditions and for the beam quality.

Results & Conclusion: Systematic energy-dependent differences between dose readings with and without the B0 field were observed. For the Semiflex 0.3, the measured dose in the presence of the B0 field increased by 0.75%, 0.58%, 0.59% and 0.41% for 70 MeV, 110 MeV, 150 MeV and 190 MeV, respectively, and decreased by 0.36% for 226.7 MeV. For the same energies, the dose measured by the PinPoint3D increased by 0.81%, 0.69%, 0.63%, 0.47% and 0.31%. For all energies, the standard deviations were smaller than 0.10% and 0.19% for the Semiflex 0.3 and PinPoint3D IC, respectively.
Beam energy-dependent correction of IC readings is mandatory and appears feasible for accurate proton beam dosimetry in magnetic fields.