1. Heavy-Ion Medical Center, Gunma University, 3-39-22, Showa-machi, Maebashi 371-8511, Gunma, Japan
2. Department of Radiation Oncology, Gunma University Graduate School of Medicine, 3-39-22, Showa-machi, Maebashi 371-8511, Gunma, Japan
3. Department of Radiation Oncology, Shonan Kamakura General Hospital, 1370-1, Okamoto, Kamakura 247-8533, Kanagawa, Japan
4. Department of Radiation Oncology, Gunma Prefectural Cancer Center, 617-1, Takabayashi-nishi, Ota 373-8550, Gunma, Japan