1. Department of Oncology Radiotherapy, Yantaishan Hospital, Yantai, Shandong 264000, P.R. China
2. Department of Oncology, People's Hospital of Haiyang, Haiyang, Shandong 265100, P.R. China
3. Department of Oncology Radiotherapy, Yantai Affiliated Hospital of Binzhou Medical University, Yantai, Shandong 264100, P.R. China