1. Department of Radiation Oncology, Hokkaido University Hospital , Kita14, Nishi5, Kita-Ku, Sapporo, Hokkaido 060-8648 , Japan
2. Department of Radiation Oncology, Kin-ikyo Chuo Hospital , 1-9-1 Higashinaebo5, Higashi-ku, Sapporo, Hokkaido 007-0805 , Japan
3. Department of Radiation Oncology , Faculty and Graduate School of Medicine, , Kita14, Nishi5, Kita-Ku, Sapporo, Hokkaido 060-8648 , Japan
4. Hokkaido University , Faculty and Graduate School of Medicine, , Kita14, Nishi5, Kita-Ku, Sapporo, Hokkaido 060-8648 , Japan
5. Department of Otolaryngology-Head and Neck Surgery , Faculty and Graduate School of Medicine, , Kita14, Nishi5, Kita-Ku, Sapporo, Hokkaido 060-8648 , Japan
6. Department of Radiation Medical Science and Engineering , Faculty and Graduate School of Medicine, , Kita14, Nishi5, Kita-Ku, Sapporo, Hokkaido 060-8648 , Japan
7. Global Center for Biomedical Science and Engineering, Hokkaido University Faculty of Medicine and Graduate School of Medicine , Kita15, Nishi7, Kita-Ku, Sapporo, Hokkaido 060-8638 , Japan