1. Department of Neurosurgery, Graduate School of Biomedical and Health Sciences, Hiroshima University, Hiroshima 734-8551, Japan
2. Department of Neurosurgery, Shimane Prefectural Central Hospital, Izumo 693-8555, Japan
3. Department of Cancer Prevention Center, Chiba Cancer Center Research Institute, Chiba 260-8717, Japan
4. Department of Neurosurgery, Gunma University Graduate School of Medicine, Maebashi 371-8511, Japan
5. Department of Clinical Epidemiology and Health Economics, School of Public Health, The University of Tokyo, Tokyo 113-0033, Japan