1. Division of Radiation Oncology and Particle Therapy, National Cancer Center Hospital East, Kashiwa, Chiba, Japan;
2. High Energy Accelerator Research Organization, Tsukuba, Japan;
3. Department of Radiation Physics, The University of Texas M.D. Anderson Cancer Center, Houston, TX, USA;
4. Division of Translational Research, EPOC, National Cancer Center, Kashiwa, Chiba, Japan