1115 Background: Young women with metastatic breast cancer (MBC) represent a distinct and understudied population and often present with aggressive disease features. However, population-level studies characterizing clinicopathologic features, treatment patterns, and outcomes in this group remain limited. Methods: The National Cancer Database (NCDB) was queried to identify women diagnosed with MBC. Patients were stratified by age (<40 vs ≥40 years). Baseline characteristics were compared using chi-square tests. Overall survival (OS) was assessed using Kaplan–Meier analysis and compared using log-rank tests. Multivariable Cox regression was performed to evaluate independent associations with OS. Results: Among 59,213 women with MBC, 4,406 (7.4%) were aged <40 years. Median follow-up was 25.3 months (IQR 9.3–42.1). Compared with women ≥40, young women had more grade III tumors (61.0% vs 50.4%), lymphovascular invasion (44.2% vs 31.7%), and liver metastases (35.3% vs 23.5%). Young women were also more likely to receive chemotherapy (44.6% vs 21.7%), immunotherapy (44.2% vs 27.2%), radiation (39.3% vs 29.8%), and surgery of the primary tumor (26.7% vs 13.3%), whereas receipt of hormonal therapy was lower compared to their older counterparts (56.9% vs 54.8%). Receptor subtypes in young women were distributed as follows: HR+/HER2+ (22.6%), HR+/HER2- (50.8%), HR-/HER2+ (11.0%) and TNBC (15.5%). Young women demonstrated longer median OS compared with those ≥40 (68.7 months 95% CI 65.2–74.6 vs 37.1 months 95% CI 36.4–37.7). TNBC had the worst OS among receptor subtypes (p<0.0001), with more than three-fold mortality risk compared with HR+/HER2- (HR 3.65, 95% CI 3.22–4.15). Metaplastic histology was associated with inferior OS compared to IDC (HR 3.11, 95% CI 1.72–5.64), and Black race was associated with worse OS compared to White race (HR 1.65, 95% CI 1.47–1.87). Receipt of radiation (HR 0.75, 95% CI 0.67–0.84), immunotherapy (HR 0.59, 95% CI 0.53–0.66), hormonal therapy (HR 0.42, 95% CI 0.38–0.47), and surgery to the primary site (HR 0.42, 95% CI 0.37–0.49) were associated with improved OS, whereas chemotherapy was not associated with OS. Most patients had a single metastatic site (55.5%), among which bone was the most common site (59.8%), followed by liver (18.4%), lung (7.7%), and brain (1.6%). Compared to bone-only metastases, brain (HR 2.04, 95% CI 1.17–3.56), lung (HR 2.08, 95% CI 1.58-2.74), and distant lymph node involvement (HR 1.54, 95% CI 1.18–2.00) were associated with worse OS. Conclusions: In this largest population-level study to date, young women with MBC presented with more aggressive disease features, yet demonstrated better OS compared with older patients. Survival outcomes were associated with receptor subtype, histology, and metastatic site. These findings provide valuable insights to guide clinical management of young women with MBC.
Foo et al. (2026) studied this question.