1. Division of Medical Radiation Physics and Department of Radiation Oncology; Inselspital Bern University Hospital and University of Bern; Berne 3010 Switzerland
2. Department of Radiation Oncology; University of Virginia Health System; Charlottesville 22908 Virginia USA
3. Institute for Biomedical Engineering; Swiss Federal Institute of Technology (ETH); Zurich 8092 Switzerland
4. Paul Scherrer Institute (PSI); Villigen 5232 Switzerland