1. Department of Neurosurgery, Sanbo Brain Hospital; Capital Medical University; No. 50 Yikesong Road, Xiangshan Ave Beijing 100093 P. R. China
2. Department of Neurosurgery; Chinese People's Liberation Army General Hospital; Beijing 100853 P. R. China
3. School of Public Health; Peking University; Beijing 100191 P. R. China
4. Tate Cancer Center, Baltimore Washington Medical Center; University of Maryland; Glen Burnie MD 21061 USA
5. Department of Orthopedics, Beijing Ditan Hospital; Capital Medical University; Beijing 10015 P. R. China
6. School of Basic Medical Science; Capital Medical University; Beijing 10069 P. R. China