1. Department of Radiation Oncology, Ansan Hospital Korea University Medical College, Ansan, Republic of Korea;
2. Division of Pulmonary Sleep and Critical Care Medicine, Department of Internal Medicine, Ansan Hospital Korea University Medical College, Ansan, Republic of Korea;
3. Department of Radiation Oncology, Anam Hospital Korea University Medical College, Seoul, Republic of Korea;
4. Department of Radiation Oncology, Guro Hospital Korea University Medical College, Seoul, Republic of Korea