Background: Cancer risks following radiotherapy have not been well characterized in terms of radiation dose. Before we have conducted a meta-analysis of studies on the excess relative risk per Gy (ERR) of second malignant neoplasm (SMN) among childhood cancer survivors, but the small number of eligible studies restricted quantitative evaluations. To solve this problem, we developed a statistical method to calculate an ERR estimate from other estimates, and conducted a meta-analysis again. Materials and Methods: We searched the PubMed database, and 21 studies were identified. ERR estimates were available in 10 studies, and for the rest 11 studies, we used the regression-based model to calculate a ERR estimate from other estimates. The overall ERR estimates of radiotherapy were calculated using a random effects model. Results and Discussion: The ERR estimates from 21 studies ranged from 0.004 to 11.7. The overall ERR was 0.45 (95%CI: 0.21, 0.97), which was far smaller than the corresponding estimate of 1.7 from the study on atomic bomb survivors exposed as young children. Heterogeneity was suggested by Cochran\u27s Q statistics (Q=188, DF=20, p<0.001). The reason for heterogeneity will be explored.WHO satellite meeting "Radiation risk assessment in pediatric healthcare" in KIDS Workshop 2009 in NIR
