1. Center for Proton Therapy, Paul Scherrer Institute, Villigen, Switzerland;
2. Department of Radiation Therapy, Rijksuniversiteit Groningen, Groningen, the Netherlands;
3. Department of Radiation Oncology, University Hospital of Bern, Bern, Switzerland;
4. Department of Radiation Oncology, University Hospital of Zürich, Zürich, Switzerland;
5. Department of Physics, ETH Zürich, Zürich, Switzerland