1. Department of Radiation Oncology, The Fourth Affiliated Hospital of China Medical University, Shenyang, Liaoning 110032, P.R. China
2. Department of Radiation Oncology, The First Affiliated Hospital of China Medical University, Shenyang, Liaoning 110001, P.R. China
3. Department of Radiation Oncology, Cancer Hospital of China Medical University, Liaoning Cancer Hospital and Institute, Shenyang, Liaoning 110042, P.R. China
4. Department of Epidemiology, China Medical University, Shenyang, Liaoning 110001, P.R. China