Figure 2: Relative dose distribution of a 180 MeV proton beam in PMMA on the film dosimeter, without (redscale) and with (bluescale) magnetic field. (a): Top view, (b): side view. Figure 3: Relative measured depth-dose curves with (solid lines) and without (dashed) magnetic field. resulting in a laterally shifted Bragg peak position. To investigate the impact of the magnetic field on the range of the beam (i.e. Bragg peak retraction), depth-dose curves were reconstructed from the film dose distributions by a radial integration method (see Supporting Appendix C and Figure 3). From these, the proton range R$_{80}$ , defined as the penetration depth of the beam at the 80 % distal end of the Bragg peak maximum, was extracted both with and without magnetic field. Bragg peak retraction was found to be below 0 . 5 mm for all studied energies. Small deviations were found between depth-dose curves with and without magnetic field in the plateau region (see Figure 3), however, repeated experiments showed that these were due to statistical setup uncertainties (see Supporting Appendix B.4) rather than the magnetic field. No magnetic-field induced change in the absolute measured dose was observed, showing the suitability of Gafchromic EBT3 film dosimeters for this experiment. 5 (a) (b)