Author contributions References References Competing Financial Interest Statement S.M.S. and A.L.H. conceptually designed the study. S.M.S., S.G., A.L. and A.L.H. planned and optimized the experiments; S.M.S, B.O., A.L., and M.B. planned and optimized the simulations. S.M.S., S.G. and A.L. conducted the experiments; S.M.S. conducted the simulations. S.M.S and S.G. performed the data analysis. S.M.S. performed the comparison between experiment and simulation. All authors contributed to data interpretation. S.M.S. drafted the manuscript of this article. All authors critically revised the article and approved the final version submitted for publication. The authors declare no competing financial interests. [1] Baumann, M. et al. Radiation oncology in the era of precision medicine. Nat. Rev. Cancer 16 , 234–249 (2016). [2] Gondi, V., Yock, T. I. & Mehta, M. P. Proton therapy for paediatric CNS tumours - improving treatment-related outcomes. Nat. Rev. Neurol. 12 , 334–345 (2016). [3] Durante, M., Orecchia, R. & Loeffler, J. S. Charged-particle therapy in cancer: clinical uses and future perspectives. Nat. Rev. Clin. Oncol. 14 , 483–495 (2017). [4] Lauterbur, P. C. Image formation by induced local interactions: examples employing nuclear magnetic resonance. Nature 242 , 190–191 (1973). [5] Lagendijk, J. J. W. et al. MR guidance in radiotherapy. Phys. Med. Biol. 59 , R349–R369 (2014). [6] Mutic, S. & Dempsey, J. The ViewRay system: Magnetic resonance-guided and controlled radiotherapy. Sem. Rad. Onc. (2014). [7] Fallone, B. G. The Rotating Biplanar Linac-Magnetic Resonance Imaging System. Semin. Radiat. Oncol. 24 , 200–202 (2014). [8] Lagendijk, J. J., Raaymakers, B. W. & van Vulpen, M. The magnetic resonance imaging-linac system. Semin. Radiat. Oncol. 24 , 207–9 (2014). [9] Keall, P. J., Barton, M. & Crozier, S. The Australian Magnetic Resonance Imaging-Linac Program. Semin. Radiat. Oncol. 24 , 203–206 (2014). [10] Oborn, B. M. et al. Future of medical physics: real-time MRI guided proton therapy. Med. Phys. 44 , e77–e90 (2017). [11] Oborn, B. M. et al. MRI Guided Proton Therapy: Pencil beam scanning in an MRI fringe field . ICTR-PHE, Geneva, February 15-19 (2016). [12] Raaymakers, B., Raaijmakers, A. J. E. & Lagendijk, J. J. Feasibility of MRI guided proton therapy: magnetic field dose effects. Phys. Med. Biol. 53 , 5615–5622 (2008). 9