Abstract: Proton beams are widely considered a superior radiation modality compared to x-rays. However, proton therapy is a limited resource that is not available to every patient who may benefit from it. In our work, we explore combined proton-photon treatments with the goal of 1) optimally make use of a limited number of proton therapy slots and 2) develop a concept for wide-spread implementation of cost-effective proton therapy without a gantry. To that end, we consider a fixed horizontal proton beamline integrated into a conventional treatment room, so that both protons and photons are delivered in each fraction.