1. Departments of Radiation Oncology and Medical Biophysics, Princess Margaret Cancer Center, University Health Network, Toronto, Ontario, Canada
2. Department of Medical Biophysics, British Columbia Cancer Research Centre, Vancouver, British Columbia, Canada
3. Department of Radiation Oncology, Massachusetts General Hospital, Boston, Massachusetts
4. Roberts Proton Therapy Center, Department of Radiation Oncology, University of Pennsylvania, Philadelphia, Pennsylvania
5. Department of Physics, Harvard University, Boston, Massachusetts