1. Department of Physics and Technology, University of Bergen, Bergen, Norway;
2. Department of Physics, University of Oslo, Oslo, Norway;
3. Department of Oncology and Medical Physics, Haukeland University Hospital, Bergen, Norway;
4. Centro Nazionale di Adroterapia Oncologica (CNAO Foundation), Pavia, Italy;
5. Heidelberg Ion Beam Therapy Center (HIT), Heidelberg, Germany;
6. Department of Medical Physics, Oslo University Hospital, Oslo, Norway