Proton beam therapy (PBT) is a potential new alternative to treatment with 
photon radiotherapy that may reduce the risk of late toxicity and secondary 
cancer, especially for pediatric tumors. The goal of this study was to evaluate 
the long-term benefits of PBT in cancer survivors. A retrospective observational 
study of pediatric patients who received PBT was performed at four institutions 
in Japan. Of 343 patients, 62 were followed up for 5 or more years. These 
patients included 40 males and 22 females, and had a median age of 10 years 
(range: 0-19 years) at the time of treatment. The irradiation dose ranged from 
10.8 to 81.2 GyE (median: 50.4 GyE). The median follow-up period was 8.1 years 
(5.0-31.2 years). The 5-, 10- and 20-year rates for grade 2 or higher late 
toxicities were 18%, 35% and 45%, respectively, and those for grade 3 or higher 
late toxicities were 6%, 17% and 17% respectively. Univariate analysis showed 
that the irradiated site (head and neck, brain) was significantly associated 
with late toxicities. No malignant secondary tumors occurred within the 
irradiated field. The 10- and 20-year cumulative rates for all secondary tumors, 
malignant secondary tumors, and malignant nonhematologic secondary tumors were 
8% and 16%, 5% and 13%, and 3% and 11%, respectively. Our data indicate that PBT 
has the potential to reduce the risk of late mortality and secondary malignancy. 
Longer follow-up is needed to confirm the benefits of PBT for pediatric tumors.