Background: Secondary cancers account for 16% of all new cancer diagnoses, with breast cancer (BC) the most common secondary cancer. We have shown that secondary BC has unique characteristics and decreased survival compared with primary BC in adolescent and young adults (AYA; 15–39 years old). However, older BC populations are less well studied. Methods: Females (age ≥ 15 years) diagnosed with primary BC during 1991–2015 (n = 377,167) and enrolled in the California Cancer Registry were compared with those with secondary BC (n = 37,625) by age (15–39, 40–64, ≥ 65 years). We examined BC-specific survival (BCSS) accounting for other causes of death as a competing risk using multivariable Cox proportional hazards regression. Results: Most secondary BC patients were of older age (15–39, n = 777; 40–64, n = 15,848; ≥ 65, n = 21,000). Compared with primary BC treatment, secondary BCs were more often treated with mastectomy and less often with chemotherapy and/or radiation. BCSS was shorter in secondary BC patients than primary BC patients, but the survival difference between secondary and primary BC diminished with age [15–39 hazard ratio (HR): 2.09, 95% confidence interval (CI) 1.83–2.39; 40–64 HR: 1.51; 95% CI 1.44–1.58; ≥ 65 HR: 1.14; 95% CI 1.10–1.19]. Survival differences were most pronounced in women with hormone receptor positive disease and Hispanic and Asian/Pacific Islanders 40–64 years of age. Conclusions: When BC is diagnosed following a prior cancer of any organ site, BCSS is worse than when compared with patients for whom BC is the primary diagnosis, suggesting that we may need to tailor our treatments for women with secondary BC.
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