1. University Medical Center Utrecht, Radiation Oncology, Utrecht, the Netherlands
2. Radboud University Medical Center, Radiation Oncology, Nijmegen, the Netherlands
3. The Netherlands Cancer Institute, Radiation Oncology, Amsterdam, the Netherlands
4. University Hospitals Leuven, Radiation Oncology, Leuven, Belgium
5. Julius Center for Health Sciences and Primary Care, University Medical Center Utrecht, Utrecht University, Utrecht, the Netherlands
6. Holland Proton Therapy Center, Delft, the Netherlands
7. Limburgs Oncologisch Centrum, Jessa Ziekenhuis, Radiation Oncology, Hasselt, Belgium