Incidence and risk: The epidemiology and long-term outcomes of early-onset luminal B breast cancer.

e12753 Background: Early-onset breast cancer is an increasing public health concern, with younger women disproportionately affected by aggressive subtypes such as Luminal B disease. Despite advances in systemic therapy, population-level data on long-term outcomes in this subgroup remain limited. We conducted a population-based study using the SEER Incidence 17 Registries database (November 2024 submission; 2000–2022) accessed via SEER*Stat v9.0.42.2. Female patients with invasive Luminal B breast cancer were identified using the SEER molecular subtype variable and stratified by age at diagnosis (18–39). Age-adjusted incidence rates were calculated and trended over time using Joinpoint v5.4.0. Five- and ten-year breast cancer–specific survival were estimated using SEER survival sessions to evaluate temporal patterns in short- and long-term outcomes. Methods: Using SEER Incidence 17 Registries (2000–2022), we evaluated incidence and 5- and 10-yearcause-specific survival among women aged 15–39 diagnosed with Luminal B breast cancer.Analyses were conducted using SEER*Stat v9.0.42.2, with temporal trends assessed viaJoinpoint v5.4.0. Results: Using SEER 17 registries (2010–2022), incidence of Luminal B breast cancer among women aged 15–39 increased across all age strata. The highest absolute incidence was observed in women aged 35–39, with a significant rise through 2014 followed by plateauing thereafter, while younger cohorts (20–34) demonstrated steady, incremental increases over the study period. Survival analyses revealed divergent temporal patterns. Five-year breast cancer–specific survival modestly improved across age groups diagnosed between 2010 and 2017, most notably among women aged 25–29 and 30–34, consistent with advances in systemic therapy and endocrine optimization. In contrast, ten-year cause-specific survival declined among women aged 30–34 and 35–39 diagnosed between 2010 and 2012, despite stable or improving short-term outcomes. This discordance suggests that early survival gains have not translated into durable long-term benefit, highlighting persistent late recurrence risk in young patients with Luminal B disease. Collectively, these findings demonstrate a growing incidence of early-onset Luminal B breast cancer accompanied by short-term survival improvements but worsening long-term outcomes in key age subgroups. Conclusions: Once thought uncommon, early-onset Luminal B breast cancer is increasing in incidence, with modest improvements in short-term survival but persistent declines in long-term outcomes. These population-level findings suggest that contemporary therapies improve early disease control without ensuring durability, underscoring the need for policies and research strategies focused on long-term risk reduction and survivorship in young women.