Breast cancer incidence is rising among young Asian women, who are still underrepresented in international datasets. We examined the epidemiology and clinical outcomes of Asian patients under 40 years of age across various breast cancer subtypes to highlight their unmet clinical needs. We evaluated women aged 20 years and older diagnosed with early-stage breast cancer from the prospective Asian Breast Cancer Cooperative Group (ABCCG) cohort. For reference, data from the SEER cancer registry were included. Patients were divided into three age categories: young (<40 years), likely premenopausal mid-age (40–49 years), and likely postmenopausal (≥50 years). Survival analyses were conducted using multivariable Cox proportional hazards models, adjusting for tumor subtype, T stage, lymph node involvement, histologic grade, and study site. This study analyzed 45,021 breast cancer patients from Asian centers, alongside 496,332 SEER-White and 18,279 SEER-Asian patients. The median age at diagnosis was notably lower in the Asian cohort (51 years) compared with SEER-Asians (58 years; P < 0.0001) and SEER-Whites (62 years). In the young-age subgroup, HR+/HER2− breast cancers were more frequently observed among Asians and SEER-Asians than SEER-Whites (61.2% and 59.8% vs. 54.7%). Within the Asian cohort, young women with HR+/HER2− tumors had poorer overall survival relative to the mid-age group (6-year OS: 94.4% vs. 96.6%; HR 0.62; P < 0.001). A similar pattern was seen in SEER-Whites, where younger patients experienced an earlier drop in survival compared with those aged 40–49 (89.1% vs. 94.0%; P < 0.001).Among ABCCG-Asian patients under 40 with HR+/HER2− breast cancer, survival outcomes were worse compared with those in the mid-age group. These findings emphasize the adverse prognosis in younger patients and highlight the importance of personalized treatment strategies, including ovarian function suppression, with consideration of ethnic-specific factors.
