1. Centre for Proton Therapy, Paul Scherrer Institute, ETH Domain, 5232 Villigen, Switzerland
2. Faculty of Medicine, University of Zurich, 8006 Zurich, Switzerland
3. Department of Biostatistics at Epidemiology, Biostatistics and Prevention Institute, University of Zurich, 8001 Zurich, Switzerland
4. Department of Radiation Oncology, Inselspital, Bern University Hospital, University of Bern, 3010 Bern, Switzerland
5. Department of Radiation Oncology, University Hospital of Zurich, University of Zurich, 8091 Zurich, Switzerland