1. Department of Radiation Oncology, University of Tsukuba, Tsukuba 305-8576, Ibaraki, Japan
2. Department of Radiation Oncology, Tsukuba Medical Center Hospital, Tsukuba 305-8558, Ibaraki, Japan
3. Department of Pediatric Radiation Therapy Center/Pediatric Proton Beam Therapy Center, Hebei Yizhou Cancer Hospital, Zhuozhou 072750, China
4. Department of Pediatrics, University of Tsukuba Hospital, Tsukuba 305-8575, Ibaraki, Japan
5. Department of Child Health, Institute of Medicine, University of Tsukuba, Tsukuba 305-8575, Ibaraki, Japan
6. Department of Biostatistics, Institute of Medicine, University of Tsukuba, Tsukuba 305-8575, Ibaraki, Japan