Recent progress in the treatment for pediatric malignancies using a combination 
of surgery, chemotherapy, and radiotherapy has improved survival. However, late 
toxicities of radiotherapy are a concern in long-term survivors. A recent study 
suggested reduced secondary cancer and other late toxicities after proton beam 
therapy (PBT) due to dosimetric advantages. In this study, we evaluated the 
safety and efficacy of PBT for pediatric patients treated in Japan. A 
retrospective observational study in pediatric patients who received PBT was 
performed. All patients aged <20 years old who underwent PBT from January 1983 
to August 2014 at four sites in Japan were enrolled in the study. There were 343 
patients in the study. The median follow-up periods were 22.6 months 
(0.4-374.3 months) for all patients and 30.6 months (0.6-374.3 months) for 
survivors. The estimated 1-, 3-, 5-, and 10-year survival rates were 82.7% (95% 
CI: 78.5-87.0%), 67.4% (61.7-73.2%), 61.4% (54.8-67.9%), and 58.7% (51.5-65.9%), 
respectively. Fifty-two events of toxicity ≥ grade 2 occurred in 43 patients. 
Grade 4 toxicities of myelitis, visual loss (two cases), cerebral vascular 
disease, and tissue necrosis occurred in five patients. This study provides 
preliminary results for PBT in pediatric patients in Japan. More experience and 
follow-up with this technique are required to establish the efficacy of PBT in 
this patient population.