Figure 5: Top panel: Predicted (green) and measured (copper) dose distribution of a 160 MeV proton beam in PMMA without (a) and with magnetic field (b). Dose is given in percent relative to the median dose in the central plateau region (1 . 9 cm ≤ x ≤ 2 . 1 cm and − 0 . 5 cm ≤ y ≤ 0 . 5 cm). Figure 6: Predicted and measured Bragg peak displacement in lateral (solid) and longitudinal (dashed) direction for discrete beam energy levels. Error bars refer to systematic and statistical uncertainties given in Supplement Tables A.1 and B.4. Individual data points are interconnected for visualization only. towards the Bragg peak. The difference in the absolute Bragg peak dose was consistent with the findings of previous studies [19, 24, 25], and was not increased by the magnetic field. Thus, the introduction of a magnetic field did not compromise the dose prediction accuracy. To quantify the accuracy of the calculated Bragg peak position, its retraction and deflection were com- pared (see Figure 6). Both quantities agreed within measurement and simulation uncertainties (0 . 8 mm) for all studied energies. There was no systematic difference in Bragg peak retraction and deflection between the simulation and measurement data. For the first time, magnetic field induced dose distortions of a slowing-down therapeutic proton beam in a tissue-like medium have been measured and compared against particle tracking simulations. In a transverse magnetic field of 0 . 95 T, it was shown that lateral Bragg peak displacement ranges between 1 mm and 10 mm for proton energies between 80 and 180 MeV in PMMA. Range retraction was found to be ≤ 0 . 5 mm, which Conclusions (a) (b) 7