Dosimetric evidence confirms computational model of magnetic field induced dose distortions of therapeutic proton beams Sonja M. Schellhammer a,b, $^{∗}$, Sebastian Gantz $^{a,b}$, Armin L¨ uhr $^{a,b,c}$, Bradley M. Oborn$^{d,e}$, Michael Bussmann $^{f}$, Aswin L. Hoffmanna,b,g $^{b}$Helmholtz-Zentrum Dresden-Rossendorf, Institute of Radiooncology - OncoRay, H¨ andelallee 26, 01309 Dresden, Germany $^{c}$German Cancer Consortium DKTK, Partner Site Dresden, Dresden, Germany $^{d}$Centre for Medical Radiation Physics, University of Wollongong, Wolllongong NSW 2522, Australia $^{e}$Illawarra Cancer Care Centre, Wollongong Hospital, Wollongong NSW 2500, Australia $^{f}$Helmholtz-Zentrum Dresden-Rossendorf, Institute of Radiation Physics, Dresden, Germany $^{g}$Department of Radiation Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universit¨ at Dresden Given the sensitivity of proton therapy to anatomical variations, this cancer treatment modality is expected to benefit greatly from integration with magnetic resonance (MR) imaging. One of the obstacles hindering such an integration are strong magnetic field induced dose distortions. These have been predicted in simula- tion studies, but no experimental validation has been performed so far. Here we show the first measurement of planar distributions of dose deposited by therapeutic proton pencil beams traversing a one-Tesla transver- sal magnetic field while depositing energy in a tissue-like phantom using film dosimetry. The lateral beam deflection ranges from one millimeter to one centimeter for 80 to 180 MeV beams. Simulated and measured deflection agree within one millimeter for all studied energies. These results proof that the magnetic field induced proton beam deflection is both measurable and accurately predictable. This demonstrates the feasi- bility of accurate dose measurement and hence validates dose predictions for the framework of MR-integrated proton therapy. With about half of all cancer patients undergoing radiation therapy, this modality has become one of the three pillars of modern cancer treatment next to chemotherapy and surgery [1]. Radiation therapy makes use of ionising radiation, usually high-energetic photons, in order to irreversibly damage the DNA and chromatin of clonogenic tumour cells, and thereby prevent these cells from proliferating. Over the last two decades, the use of accelerated protons has emerged as an important treatment option, especially for tumours of the central nervous system and for paediatric cases [2, 3]. As compared to conventional photon- $^{∗}$Corresponding author Email address: s.schellhammer@hzdr.de (Sonja M. Schellhammer) Preprint submitted to arXiv.org September 7, 2017 arXiv:1709.00373v2 [physics.med-ph] 6 Sep 2017 Abstract $^{a}$OncoRay - National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universit¨ at Dresden, Helmholtz-Zentrum Dresden - Rossendorf, Dresden, Germany