Background: The purpose was to describe the development of subsequent malignant neoplasms (SMN) among a population-based cohort of pediatric cancer patients, with a focus on SMNs that occurred within the first 5 years from diagnosis.

Methods: The cohort was identified from POGONIS, an active provincial registry. Cohort members were Ontario residents ages 0 to 14.9 years at primary diagnosis between January 1985 and December 2008. SMNs that developed <18 years were captured by POGONIS, whereas SMNs diagnosed later were identified through linkage. Cumulative incidence and standardized incidence ratios (SIR) were calculated, and proportional hazards models were estimated to examine factors associated with SMN development.

Results: A total of 7,920 patients were eligible. 2.4% (188/7,920) developed 197 SMNs. Mean follow-up time was 10.7 years (SD = 7.6 years; range, 0.0–26.4 years) with mean time to SMN of 8.5 years (SD = 6.3 years; range, 0.0–24.9 years). The SIR for the development of a SMN was 9.9 [95% confidence interval (CI), 8.6–11.4]. 40.6% of SMNs (80/197) developed within 5 years. Early SMNs were more likely to be leukemia and lymphoma. Factors associated with early SMN were primary diagnosis of a bone tumor (OR, 4.88; 95% CI, 1.52–15.60), exposure to radiotherapy (OR, 1.82; 95% CI, 1.02–3.22), and the highest dose of epipodophyllotoxin (OR, 3.74; 95% CI, 1.88–7.42).

Conclusions: Over 40% of SMNs diagnosed in childhood cancer patients occurred in the first 5 years after diagnosis, suggesting a need for early and ongoing surveillance.

Impact: The early development of certain SMNs reinforces the need for early and continued surveillance at all stages for pediatric cancer patients. Cancer Epidemiol Biomarkers Prev; 24(10); 1–8. ©2015 AACR.

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