1 Department of Radiation Oncology, Graduate School of Medicine, Juntendo University, Tokyo 113-8421, Japan.
2 Department of Radiology, Juntendo University Hospital, Tokyo, Japan.
3 Department of Otorhinolaryngology, Head and Neck Surgery, Graduate School of Medicine, Juntendo University, Tokyo 113-8421, Japan.
4 Department of Radiation Oncology, Graduate School of Medicine, Juntendo University, Tokyo 113-8421, Japan. Electronic address: ksasai@juntendo.ac.jp.