Risk of developing subsequent primary breast cancer among non-breast cancer survivors: a retrospective cohort study

Cancer survivors have an elevated risk of developing subsequent primary cancer, with breast cancer being one of the most common sites. Our study aimed to determine the incidence, patterns of, and risk factors for diagnosis of a subsequent primary breast cancer (SPBC) among survivors of adult non-breast cancers.

Adult females diagnosed with a non-breast cancer in Alberta, Canada between 2000 and 2021 who survived ≥ 180 days were followed until SPBC diagnosis, death, or censoring on December 31, 2021. The incidence of SPBC was compared to the cancer-free general population using standardized incidence ratios (SIRs) and incidence rate differences (IRDs). Exploratory analyses of the association between sociodemographic/clinical factors and SPBC risk was conducted according to cancer type using Fine-Gray models adjusting for competing risk of death.

The study included 59,687 female cancer survivors, with 1113 incident SPBC cases. Overall, there was an increased risk of SPBC among survivors compared to the cancer-free general population (SIR = 1.15 [95% CI: 1.09-1.22], 10-year cumulative incidence = 2.0%). Significantly elevated SPBC risk was found among survivors of kidney, endometrial, and colorectal cancers, as well as obesity-related cancers combined. The increased risk of SPBC was primarily driven by early-stage and hormone receptor positive diagnoses. Depending on the primary cancer site, risk factors for SPBC included overweight/obese body mass index, diabetes, and cancer treatment.

History of cancer should prompt discussion of breast cancer awareness, screening, and risk reduction. Future research is needed to inform personalized risk assessment for survivors of adult non-breast cancers.