Clinical and biological patterns of breast cancer recurrence in young women: A US population-based study.

e12571 Background: Breast cancer (BC) in young women is characterized by distinct clinical and molecular features, frequently presenting with more aggressive subtypes and poorer outcomes compared to older patients. While advances in treatment have improved survival rates, recurrence remains a significant and understudied challenge in this population. This study aims to study the clinical patterns in recurrent BC in young women using the Surveillance, Epidemiology, and End Results (SEER) database. Methods: The SEER database was first queried to identify females with BC below age 40 from 2000- 2021. Recurrence was determined using SEER site recode-breast as a subsequent event. Demographic and clinical variables, including age, race, stage, receptor status, and treatment, were analyzed. Statistical methods included chi-square tests, Kaplan-Meier survival curves, and Cox proportional hazards models. Analyses were performed using R (v4.4.1). Results: A total of 2,542 young women with BC were identified (primary cohort), with 2,895 recurrence events. In the primary cohort, 60.6% were aged 35–39, while recurrent cases predominantly occurred in the 40–44 (28.8%) and 45–49 (23.7%) age ranges. Most patients were White (69% primary, 68.5% recurrent), followed by African American (20.5% primary, 20.9% recurrent) and Asian/Pacific Islander individuals (~10% both cohorts). Receptor profile analysis revealed high ER positivity in both primary (55.5%) and recurrent cases (55.8%). HER2 positivity was significantly higher in recurrent cases (37.7%) compared to the primary cohort (12.2%), as was TNBC prevalence (16.7% vs. 3.5%). T2 staging was most frequent in primary cases (33.7%), whereas recurrent cases showed a shift toward smaller tumors (T1c, 15%) but with increased incidence of metastasis (7.4% vs. 5.5%). Cox regression analysis highlighted several predictors of survival in recurrent young BC. Younger patients (25–29 years) had a higher, albeit statistically nonsignificant mortality rate (HR 1.64, p=0.50). Advanced tumor (T4d HR 4.10, p=0.06) and nodal (N3 HR 3.19, p<0.001) stages were associated with worse survival outcomes. ER (HR 0.59, p<0.001) and PR positivity (HR 0.52, p<0.001) were associated with lower mortality. HER2 positivity (HR 1.28, p=0.16) was associated with worse outcomes however the results did not reach statistical significance. Conclusions: This study underscores the aggressive biological profile of recurrent BC in young women, with a higher prevalence of HER2-positive and TNBC subtypes. While ER and PR positivity predicted improved survival; advanced T, N stages and HER 2 positivity were associated with worse outcomes. These findings offer valuable insights for tailoring interventions and addressing recurrence-related challenges in young BC patients.