1. Department of Radiation Oncology, Narita Memorial Proton Center, 78 Shirakawa-cho, Toyohashi 441-8021, Japan
2. Medical Physics Laboratory, Division of Health Science, Graduate School of Medicine, Osaka University, 1-7 Yamadaoka, Suita-shi 565-0871, Japan
3. Department of Radiology, Graduate School of Medical Sciences, Nagoya City University, 1 Kawasumi, Mizuho-cho, Mizuho-ku, Nagoya 467-8601, Japan