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Background: Breast cancer survivors are living longer due to early detection and advances in treatment and are at increased risk for second primary cancers. Comprehensive evaluation of second cancer risk among patients treated in recent decades is lacking. Methods: We identified 16,004 females diagnosed with a first primary stage I-III breast cancer between 1990 and 2016 (followed through 2017) and survived ≥ 1 year at Kaiser Permanente (KP) Colorado, Northwest, and Washington. Second cancer was defined as an invasive primary cancer diagnosed ≥ 12 months after the first primary breast cancer. Second cancer risk was evaluated for all cancers (excluding ipsilateral breast cancer) using standardized incidence ratios (SIRs), and a competing risk approach for cumulative incidence and hazard ratios (HRs) adjusted for KP center, treatment, age, and year of first cancer diagnosis. Results: Over a median follow-up of 6.2 years, 1,562 women developed second cancer. Breast cancer survivors had a 70% higher risk of any cancer (95%CI = 1.62–1.79) and 45% higher risk of non-breast cancer (95%CI = 1.37–1.54) compared with the general population. SIRs were highest for malignancies of the peritoneum (SIR = 3.44, 95%CI = 1.65–6.33), soft tissue (SIR = 3.32, 95%CI = 2.51–4.30), contralateral breast (SIR = 3.10, 95%CI = 2.82–3.40), and acute myeloid leukemia (SIR = 2.11, 95%CI = 1.18–3.48)/myelodysplastic syndrome (SIR = 3.25, 95%CI = 1.89–5.20). Women also had elevated risks for oral, colon, pancreas, lung, and uterine corpus cancer, melanoma, and non-Hodgkin lymphoma (SIR range = 1.31–1.97). Radiotherapy was associated with increased risk for all second cancers (HR = 1.13, 95%CI = 1.01–1.25) and soft tissue sarcoma (HR = 2.36, 95%CI = 1.17–4.78), chemotherapy with decreased risk for all second cancers (HR = 0.87, 95%CI = 0.78–0.98) and increased myelodysplastic syndrome risk (HR = 3.01, 95%CI = 1.01–8.94), and endocrine therapy with lower contralateral breast cancer risk (HR = 0.48, 95%CI = 0.38–0.60). Approximately 1 in 9 women who survived ≥ 1 year developed second cancer, 1 in 13 developed second non-breast cancer, and 1 in 30 developed contralateral breast cancer by 10 years. Trends in cumulative incidence declined for contralateral breast cancer but not for second non-breast cancers. Conclusions: Elevated risks of second cancer among breast cancer survivors treated in recent decades suggests that heightened surveillance is warranted and continued efforts to reduce second cancers are needed. [ABSTRACT FROM AUTHOR]