1. Division of Radiation Oncology, Kobe University Graduate School of Medicine, Kobe 650-0017, Japan
2. Department of Otolaryngology-Head and Neck Surgery, Kobe University Graduate School of Medicine, Kobe 650-0017, Japan
3. Kobe University Hospital Cancer Center, Kobe 650-0017, Japan
4. Department of Radiation Oncology, Andrew Love Cancer Centre, Barwon Health, Geelong, VIC 3220, Australia
5. Division of Radiation Oncology, Tottori University, Yonago 680-0945, Japan