Corresponding Authors: Su-Xia Han, Department of Radiation Oncology, First Affiliated Hospital, Xi'an Jiaotong University, Xi'an, Shaanxi 710061, P. R. China. Email: hansuxia2.2099@yahoo.com.cn. Corresponding Authors: Be Xu, Department of Radiation Oncology, The Methodist Hospital Research Institute, 6670 Bertner Avenue, Houston, TX 77030, USA. Tel: +1-713-441-3996; Fax: +1-713-441-4493; Email: bxu@tmhs.org.