The contribution of second primary cancers to the mortality of patients with a first primary breast cancer

Second primary cancers (SPCs) are estimated to affect nearly 5% of patients with breast cancer within 10 years of their diagnosis. This study aimed to estimate the contribution of SPCs to the mortality of patients with a breast first primary cancer (FPC).

A population-based cohort of 17,210 patients with a breast FPC diagnosed between 2000 and 2010 was followed for SPCs (31/12/2015) and vital status (30/06/2021). Patients diagnosed with an SPC (265 synchronous and 897 metachronous, ≤ 1 and > 1 year after the FPC, respectively) were matched (1:3, by five-year age group and year of breast FPC diagnosis) to those without an SPC and alive when the corresponding SPC was diagnosed.

Significantly higher hazards of death were found among patients with an SPC [hazard ratio of 1.56, 95% confidence interval (CI) 1.29–1.89 for synchronous SPCs; and 2.85, 95%CI 2.56–3.17 for metachronous SPCs] compared to patients with a breast FPC only. Estimates were higher for synchronous lung, stomach, non-Hodgkin lymphoma and breast SPCs, and metachronous liver, stomach, ovary, lung, rectum, corpus uteri, colon, breast, and non-Hodgkin lymphoma SPCs. The 15-year cumulative mortality was 59.5% for synchronous SPCs and 68.7% for metachronous SPCs, which was higher than in patients with a breast FPC only (43.6% and 44.8%, respectively).

In Northern Portugal, patients with an SPC following a breast FPC have a higher mortality compared with patients with a breast FPC only.