Many methods exist to improve treatment outcome in radiotherapy. Two of these are image-guided radiotherapy (IGRT) and proton therapy. IGRT aims at a more precise delivery of the radiation, while proton therapy is able to achieve more conformal dose distributions. In order to maximally exploit the sharp dose gradients from proton therapy it has to be combined with soft-tissue based IGRT. MRI-guided photon therapy (currently under development) offers unequalled soft-tissue contrast and real-time image guidance. A hybrid MRI proton therapy system would combine these advantages with the advantageous dose steering capacity of proton therapy. This paper addresses a first technical feasibility issue of this concept, namely the impact of a 0.5 T magnetic field on the dose distribution from a 90 MeV proton beam. In contrast to photon therapy, for MR-guided proton therapy the impact of the magnetic field on the dose distribution is very small. At tissue-air interfaces no effect of the magnetic field on the dose distribution can be detected. This is due to the low-energy of the secondary electrons released by the heavy protons.