Long-term population-based risks of breast cancer after childhood cancer

Abstract

Previous studies have reported substantially increased risks of breast cancer among survivors of childhood cancer at 10-20 years posttreatment. Whether these excess risks are sustained beyond 40 years of age when general population incidence of breast cancer begins its steep increase is largely unknown. We quantified the risk of breast cancer in adult female survivors with considerably more survivors followed-up beyond 40 years of age than previously available. Standardized Incidence Ratios (SIR), Excess Absolute Risks (EAR), and cumulative incidence were calculated within a population-based cohort of 8,093 female survivors of childhood cancer. Poisson regression models were used to model SIRs and EARs in a multivariable setting. Eighty-one survivors developed a primary breast cancer, where 37.5 were expected (SIR= 2.2, 95% CI: 1.7-2.7). SIRs decreased significantly with increasing attained age (p(trend) < 0.001) to an SIR of 0.9 (95% CI: 0.5-1.8) at ages beyond 50 years; EARs increased significantly to about 40 years of age (p(trend) < 0.001) but then plateau. Between 30 and 49 years of age survivors experienced approximately 1 extra breast cancer per 1,000 survivors per year. Overall, 3% developed breast cancer by the age of 50. The substantially increased relative risks of breast cancer observed at 10-20 years postdiagnosis are not sustained into ages at which the risk of breast cancer in the general population becomes substantial. Among women who survived to an age of at least 50 years there is currently no evidence of an increased risk of breast cancer.