1. Proton Medical Research Center, University of Tsukuba Hospital, 2 Chome-1-1 Amakubo, Tsukuba, Ibaraki Prefecture 305-8576, Japan
2. National Institute of Radiological Sciences, National Institutes for Quantum and Radiological Science and Technology, 4-9-1 Anagawa, Inage, Chiba, Chiba 263-8555, Japan
3. Faculty of Medicine, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8575, Japan
4. Department of Radiation Oncology, The Cancer Institute Hospital of Japanese Foundation for Cancer Research, 3 Chome-8-31 Ariake, Koto, Tokyo 135-0063, Japan
5. Department of Radiology, University of Tsukuba Hospital, 2 Chome-1-1 Amakubo, Tsukuba, Ibaraki Prefecture 305-8576, Japan