1. 611 Orthopaedic Research Laboratory; Radboud Institute for Health Sciences; Radboud university medical center; P.O. Box 9101 Nijmegen 6500 HB The Netherlands
2. Radiotherapeutic Institute Friesland; Leeuwarden The Netherlands
3. Department of Radiation Oncology; University Medical Center Groningen; University of Groningen; Groningen The Netherlands
4. Department of Radiotherapy; Radboud university medical center; Nijmegen The Netherlands
5. Department of Radiotherapy; Leiden University Medical Center; Leiden The Netherlands
6. Laboratory of Biomechanical Engineering; University of Twente; Enschede The Netherlands