1. Department of Radiation Oncology, Massachusetts General Hospital, Boston, Massachusetts 02114
2. Centre for Cancer Research and Cell Biology, Queens University Belfast, Belfast, United Kingdom
3. Department of Radiation Oncology, University of California San Francisco, California 94143
4. Division of Molecular and Clinical Cancer Sciences, Faculty of Biology, Medicine and Health, University of Manchester, Manchester, United Kingdom
5. SLAC National Accelerator Laboratory, Menlo Park, California
6. Department of Engineering Physics, Tsinghua University, Beijing 100084, P.R. China