1. Department of Radiation Oncology, University of Tsukuba , 1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8575 , Japan
2. Department of Radiation Oncology, Tsukuba Medical Center Hospital , 1-3-1 Amakubo, Tsukuba, Ibaraki, 305-8558 , Japan
3. Department of Pediatrics, University of Tsukuba Hospital , 2-1-1 Amakubo, Tsukuba, Ibaraki, 305-8576 , Japan
4. Department of Child Health, Institute of Medicine, University of Tsukuba , 1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8575 , Japan
5. Department of Biostatistics, Institute of Medicine, University of Tsukuba , 1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8575 , Japan