1. Department of Radiation Oncology, College of Medicine, The Catholic University of Korea, Seoul, South Korea;
2. Department of Radiation Oncology, Yonsei University College of Medicine, Seoul, South Korea;
3. Department of Radiation Oncology, Seoul National University College of Medicine, Seoul, South Korea;
4. Cancer Research Institution, Seoul National University College of Medicine, Seoul, South Korea;
5. Institute of Radiation Medicine, Medical Research Center, Seoul National University, Seoul, South Korea;
6. Department of Otorhinolaryngology-Head and Neck Surgery, Seoul National University College of Medicine, Seoul, South Korea;
7. Proton Therapy Center, Research Institute and Hospital, National Cancer Center, Goyang, South Korea;
8. Department of Radiation Oncology, Gyeongsang National University School of Medicine and Gyeongsang National University Changwon Hospital, Jinju, South Korea;
9. Department of Radiation Oncology, Ajou University School of Medicine, Suwon, South Korea;
10. Department of Radiation Oncology, Keimyung University, School of Medicine, Daegu, South Korea;
11. Department of Radiation Oncology, School of Medicine, Kyungpook National University, Daegu, South Korea