Abstract PS5-04-01: Survival Outcomes in Metastatic Breast Cancer: An analysis of the US population-based SEER database

Abstract Background Over the last 15 years, several therapies have been approved for the treatment of metastatic breast cancer (mBC) in the United States (U.S.). Given the evolving treatment landscape, it is important to assess and understand overall survival (OS) and related predictors in the real-world setting. Methods Data were obtained from the Surveillance, Epidemiology, and End Results (SEER), which covers approximately 48% of the U.S. population. Patients diagnosed with mBC (ICD-10: C50.0 to C50.9) between 2010 and 2021 were included. Follow-up continued until death or December 2021. Overall survival (OS) and cancer-specific survival (CSS) were estimated using Kaplan-Meier methods. Median survival and 5-year survival rates were reported. Predictors of survival were assessed using multivariate Cox proportional hazards modeling. Results The cohort included 37,017 patients with mBC. The majority (58%) were aged <65 years, with a mean age of 61 years. 54% of the patients had liver or lung metastases. Prior surgery was reported for 25% of the patients. Among patients with subtypes reported (n=31,656), the proportion of patients with HR+/HER2-, HER2+ (HR+/HER2+, HR-/HER2+), and triple negative breast cancer (TNBC) subtypes were 60%, 26%, and 14% respectively. In the overall mBC cohort, median OS was 28 months, with a 5-year OS rate of 28% and median CSS was 32 months, with a 5-year CSS rate of 32% (Table 1). Among patients aged ≥65 years in the overall mBC cohort, median OS and CSS were 18 and 23 months, respectively, with 5-year survival rates of 20% and 26%. In the HR+/HER2- mBC subgroup, median OS and CSS were 35 and 39 months, with a 5-year OS rate of 30% and 34% respectively. In the HER2+ mBC subgroup, median OS and CSS were 44 and 49 months, with a 5-year OS rate of 41% and 44% respectively. In the triple negative mBC subgroup, median OS and CSS was 12 and 13 months, with a 5-year OS rate of 12% and 14% respectively. Significant predictors of OS and CSS included age, race, histology, breast cancer subtype and prior surgery status (p < 0.05). Overall survival prognosis was significantly (p<0.001) worse for TNBC compared to HR+/HER2- mBC patients (HR;95% CI:2.38;2.29,2.48). Conclusion Overall survival in patients diagnosed with mBC between 2010 to 2021 varied by breast cancer subtype with prognoses relatively worse in TNBC patients compared to HR+/HER2- patients. Continued development of novel therapies for TNBC patients will be key to achieving further improvements in survival outcomes. Citation Format: A. Niyazov, R. Sharma, S. Bilthare, S. Iyer. Survival Outcomes in Metastatic Breast Cancer: An analysis of the US population-based SEER database [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS5-04-01.