1. Department of Proton Beam Therapy, Faculty of Medicine, Dentistry and Pharmaceutical Sciences, Okayama University , 2-5-1 Shikata-cho, Kita-ku, Okayama 700-8558, Japan
2. Department of Radiology, Faculty of Medicine, Dentistry and Pharmaceutical Sciences, Okayama University , 2-5-1 Shikata-cho, Kita-ku, Okayama 700-8558, Japan
3. Department of Obstetrics and Gynecology, Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, Okayama University , 2-5-1 Shikata-cho, Kita-ku, Okayama 700-8558, Japan
4. Department of Epidemiology, Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, Okayama University , 2-5-1 Shikata-cho, Kita-ku, Okayama 700-8558, Japan
5. Department of Radiology, Tsuyama Central Hospital , 1756 Kawasaki, Tsuyama, 708-0841, Japan