Patterns of Subsequent Cancer Incidence Over Time in Patients with Breast Cancer

Background: Breast cancer survivors face a higher risk of subsequent primary cancers. This study investigated the patterns of subsequent cancer risk according to time since breast cancer diagnosis. Methods: Using data from the Surveillance, Epidemiology, and End Results Program (2000–2018), we identified a cohort of 771,681 breast cancer survivors. Standard incidence ratios (SIR) were calculated by comparing the observed with the expected number of subsequent cancers over different follow-up periods since breast cancer diagnosis. Analyses were conducted for multiple cancer types, stratified by hormone receptor status, treatment of the first breast cancer, age, and race/ethnicity. Results: Survivors experienced a 16% increased risk of subsequent cancer with the SIR continuing to increase with longer follow-up (SIR ¼ 1.04, 1.22, and 1.31 for 12–59, 60–119, and ≥120 months). This trend was driven primarily by a subsequent breast cancer, particularly among women <50 years of age, those with initial hormone receptor–negative cancer, and racial/ethnic minorities. The patterns of subsequent nonbreast cancer risk varied by type. An early-onset and sustained increased risk was observed for subsequent leukemia, thyroid cancer, soft-tissue sarcoma, melanoma, pancreatic cancer, and uterine cancer. A delayed increased risk was observed for subsequent esophageal, ovarian, oral cavity/pharyngeal, and lung cancers, whereas for small intestine, stomach, kidney, and colorectal cancers, there was a decrease after an initial increased risk. Conclusions: Patterns in subsequent cancer risk since breast cancer diagnosis differ by cancer type and characteristics of the first breast cancer.