1. Department of Radiation Oncology, Tohoku University Graduate School of Medicine , 1-1 Seiryo-machi, Aoba-ku, Sendai, Miyagi, 980-8574 , Japan
2. Radiation Oncology Center, Ofuna Chuo Hospital , Ofuna 6-2-24, Kamakura, Kanagawa 247-0056 , Japan
3. Department of Radiation Oncology, Aichi Cancer Center Hospital , Kanokoden 1-1, Chikusa-ku, Nagoya, Aichi, 464-8681 , Japan
4. Department of Radiological Technology, Keio University , Shinanomachi 35, Shinjuku-ku, Tokyo 160-8582 , Japan