1. Department of Radiation Oncology, University of Texas MD Anderson Cancer Center, Houston, TX, USA
2. Department of Oncology and Medical Physics, Haukeland University Hospital, Bergen, Norway
3. Department of Clinical Science, University of Bergen, Bergen, Norway
4. Department of Physics and Technology, University of Bergen, Bergen, Norway
5. Physics and Astronomy Department, Rice University, Houston, TX, USA
6. Department of Radiation Physics, University of Texas MD Anderson Cancer Center, Houston, TX, USA