1. Department of Basic Medical Sciences for Radiation Damages, National Institute of Radiological Sciences, 4–9-1 Anagawa, Inage, Chiba, Chiba 263–8555, Japan
2. Formerly at Center for Charged Particle Therapy, National Institute of Radiological Sciences, 4–9-1 Anagawa, Inage, Chiba, Chiba 263–8555, Japan
3. Proton Medical Research Center, Faculty of Medicine, University of Tsukuba Hospital, 2–1-1 Amakubo, Tsukuba, Ibaraki 305–8576, Japan
4. Department of Accelerator and Medical Physics, National Institute of Radiological Sciences, 4–9-1 Anagawa, Inage, Chiba, Chiba 263–8555, Japan
5. Graduate School of Comprehensive Human Sciences, University of Tsukuba, 1–1-1 Tennodai, Tsukuba, Ibaraki 305–8575, Japan