Objective: The aim of this study is to evaluate the influence of age at external beam irradiation (EBRT) on the occurrence of second primary tumors (SPTs) inside and outside the irradiation field in hereditary retinoblastoma patients.
Design: Cross-sectional study.
Participants: The study included 263 hereditary retinoblastoma patients born in The Netherlands between 1945 and 1997.
Methods: A national register-based follow-up cohort study was performed on hereditary retinoblastoma patients. Information on therapy, age at irradiation, and location of SPT was obtained from the register. The Kaplan-Meier method calculated cumulative incidences of SPT in three subgroups: irradiation before (early EBRT) and after 12 months of age (late EBRT), and no irradiation. The Mantel-Cox method determined the statistical significance of differences between the cumulative incidence curves.
Main outcome measures: Development of SPT inside and outside a precisely defined irradiation field in relation to age at irradiation. Our definition excluded pineoblastoma as SPT, because they constitute part of a trilateral retinoblastoma; in addition, they lie outside the field of irradiation.
Results: The cumulative incidence of SPT at the age of 25 years was 22% (95% confidence intervals 13%-34%) in the early EBRT group, 3% (0%-14%) in the late EBRT group, and 5% (1%-16%) in the nonirradiated group (Mantel-Cox overall: P = 0.001; between early and late EBRT, P = 0.04). However, in early irradiated patients, the incidence of SPTs inside and outside the irradiation field was similar (11%), and the difference between early and late EBRT in incidence of SPT inside the field of irradiation was less prominent than overall (11% vs. 3%: P = 0.37). Sensitivity analysis showed the results depended on the way SPT, irradiation field, and, especially, pineoblastomas are defined.
Conclusions: Hereditary retinoblastoma confers an increased risk for the development of SPT, especially in patients treated with EBRT before the age of 12 months. However, the presence of similar numbers of SPTs inside and outside the irradiation field suggests that irradiation is not the cause. In other words, this study does not show an age effect on radiation-related risk. Rather, early EBRT is probably a marker for other risk factors of SPT.