The magnet assembly consists of two magnet poles with a size of x$_{pole}$ × y$_{pole}$ × z$_{pole}$ = 20 × 15 × 5 . 9 cm 3 each and a density of 7 . 6 g cm − 3 [26]. The two poles made up of Nd$_{2}$Fe$_{14}$B are mounted on a C-shaped yoke, mounted on a mobile table. The air gap between the poles has a height of 4 cm. As opposed to a previous study for photons [27], no systematic differences in absolute dose were found between the film measurements with and without magnetic field, which may be caused by the fact that film dosimeters were oriented perpendicular to the magnetic field in this study. EBT3 Gafchromic films (Ashland, Covington, USA; lot 05201501) cut to (20 × 15) cm 2 were used to measure the planar dose distribution of the beam. They were scanned with a flat-bed document scanner (Expression 11000XL, Epson America, Long Beach, CA) in landscape orientation, transmission mode, 24-bit color mode, and a resolution of 300 dpi. Scanned images were analysed using in-house developed software written in the Python programming language (Python Software Foundation). The net optical density (netOD) was computed from the red channel pixel intensity I and background intensity I$_{0}$ for each pixel i as and converted to dose using the calibration with k$_{1}$ = 8 . 36, k$_{2}$ = 10 . 71 and k$_{3}$ = 1 . 84. The calibration had been performed in analogy to a previous study [28]. The first 30 pixels (2 . 55 mm) around film cutting edges were omitted. Pixels were converted to x - y -coordinates as follows. Pixels were scaled with the image resolution, yielding a distribution on the film plane in x ′ and y $^{′}$. This coordinate system is tilted by α = 1 ◦ around the y -axis relative to the system defined above, which yields x = cos( α ) x ′ and y = y $^{′}$. Note that x − x ′ < 0 . 02 mm within the film. The pinholes on the scanned images, whose x - y -positions are given by the manufacturer, were automatically detected on the scanned images and hence defined a translation and rotation of the dose distribution. netOD$_{i}$ = log$_{10}$ I$_{0}$ I$_{i}$ , (A.1) D$_{i}$ = k$_{1}$ (netOD$_{i}$) + k$_{2}$ (netOD$_{i}$) k$_{3}$ (A.2) Appendix A.2. Film handling and evaluation 13