Risk of primary non-breast cancer after female breast cancer by age at diagnosis

TLDR: The age at breast cancer diagnosis is an essential risk factor for being diagnosed with a new primary non–breast cancer and the level of risk for specific ages at diagnosis may hold for many years after the diagnosis.

Abstract: Background: Women diagnosed with breast cancer at young age have been shown to be at higher risk of developing a new primary cancer compared to women diagnosed at older ages, but little is known about whether adjustment for calendar year of breast cancer diagnosis, length of follow-up and/or breast cancer treatment alters the risk pattern by age. Methods: We identified 304,703 women diagnosed with breast cancer during 1943-2006 in the Cancer Registries of Denmark, Norway and Finland. Standardized incidence ratios (SIR) or relative risks (RR) of subsequent non-breast cancer by age at cancer diagnosis were calculated using Poisson regression models adjusted for country, calendar period, length of follow-up and treatment. Excess absolute risks (EAR) were also calculated. Results: The RR for all cancer sites except breast cancer decreased with increasing age both with and without adjustments. The RR and the EAR differed for each age at diagnosis category until the women reached their late 70's. Many specific cancer forms contributed to the overall risk pattern by age with endometrial cancer as one exception. Conclusions: The age at breast cancer diagnosis is an essential risk factor for being diagnosed with a new primary non-breast cancer and the level of risk for specific ages at diagnosis may hold for many years after the diagnosis. Occurrence of endometrial cancer after breast cancer seems to follow a distinct age pattern different from that seen for most other cancer types. Impact: Future studies should aim at exploring the underlying explanations for the age-related findings.