1. Department of Radiology, Sapporo Medical University School of Medicine, Chuo-ku, S1 W16, Sapporo, Hokkaido 060-8543, Japan
2. Department of Radiation Oncology, Teine Keijinkai Hospital, Sapporo, Hokkaido, Japan
3. Department of Radiology, Sapporo City General Hospital, Hokkaido, Japan