Experimental setup for first in-phantom measurements of magnetic field effects on dose distributions of proton pencil beams

Purpose: To present an experimental measurement setup for MR-integrated proton therapy (MRiPT) dosimetry studies of magnetic field-induced dose distortion effects on slowing down proton pencil beams in a tissue-equivalent phantom.

Methods: A 0.95 T NeFeB permanent dipole magnet was utilized to generate a transverse magnetic field over a 4x20x15 cm3 air gap. A PMMA slab phantom with Gafchromic EBT3 film was placed inside the air gap to measure planar dose distributions of 80-180 MeV proton pencil beams. Integrated depth-dose curves, beam trajectories, range and deflection of the Bragg peak were extracted from the films. A 3D finite-element model -- developed to generate magnetic vector field data -- was experimentally validated by Hall probe magnetometry. Repeated measurements of the magnetic field strength were performed to assess its stability during irradiation experiments.

Results: The modelled magnetic vector field data differed less than 2% from the measured data. Magnetic field-induced beam deflection was clearly observed from the planar dose distributions (Fig. 1). Integrated depth-dose curves showed a similar form in comparison to measurements without magnetic field. Lateral displacement of the Bragg peak increased with energy from 1 to 10 mm for 80 and 180 MeV, respectively (Fig. 2). Spot measurements of the magnetic field strength showed high reproducibility (sigma=3 mT) and no effects of radiation-induced degradation.

Conclusion: For the first time, a measurement setup to study magnetic field-induced proton beam dose effects in a film phantom has been realized. The method is instrumental for building and validating Monte Carlo beam models for future MRiPT concepts.