1. Nasopharyngeal Head and Neck Tumor Radiotherapy Department, Zhongshan City People's Hospital, Zhongshan, P. R. China.
2. State Key Laboratory of Oncology in South China, Guangdong Key Laboratory of Nasopharyngeal Carcinoma Diagnosis and Therapy, Guangdong Provincial Clinical Research Center for Cancer, Sun Yat-sen University Cancer Center, Guangdong 510060, P. R. China.
3. Department of Mathematics, Faculty of Science and Technology, University of Macau, Macao Special Administrative Region, P. R. China.
4. Department of Radiation Oncology, Cancer Center, the First People's Hospital of Foshan, Foshan, P. R. China.