Abstract P2-12-13: Clinico-Pathological Characteristics and Incidence Trends of Breast Cancer in Young Women: A 10-Year Single Institution Retrospective Study

Abstract Introduction: Women diagnosed with breast cancer (BC) before age 40 (YWBC) experience a substantially higher risk of recurrence and mortality than their older counterparts. The global trends in BC incidence and mortality among YWBC are scarcely described, although recent studies suggest a rise among certain high-income countries. Material and methods: We conducted a retrospective analysis including all consecutive newly diagnosed BC patients (NBC) between January 2014 and December 2023 at Hospital Clinic of Barcelona, Spain. Clinical and immunohistochemistry characteristics (estrogen receptor [ER], progesterone receptor [PR], HER2 status and Ki67 expression) determined BC subrogated subtypes: hormone receptor-positive (HR+)/HER-2 negative (HER2-), HER2-positive (HER2+) and triple negative (TN). YWBC incidence (YI) was defined as the yearly ratio between YWBC and NBC. Our primary objective was to report the YI over the years and determine if there was any variation. We also assessed incidence differences among immunohistochemical subtypes, as well as the YI in very young BC pts (≤ 35 years, VYWBC). Incidence difference across years was evaluated using the Kruskal–Wallis test, with a significance p-value <0.05. Results: A total of 262 YWBC pts were included, accounting for 9.6% of NBC over the 10-year period, ranging from a minimum of 7.38% in 2020 to a maximum of 14.02% in 2018. When analyzing YI over the years, we observed an initial increase until 2018, followed by a decline and stabilization at a steady percentage, with no statistically significant differences (p=0.44). With a median age of 37 years (range: 18 to 40 years), the majority (76.33%) were diagnosed at early stages (i.e., stage II or less). Stages II and III represented 38.50%, whereas 6.11% were metastatic at diagnosis, without significant variations throughout the period (p=0.44). Most YWBC were ER-positive (66.41%), PR-positive (53.05%), HER2-negative (62.6%), and had a Ki67 >20% (52.29%). Most tumors were HR+ (54.23%) throughout the entire period, with minimal variation across the years. The proportion of HR+ vs HR-negative (HR-) tumors remained stable over the 10 years studied (p=0.44). HER2+ tumors accounted for 23.66% of YWBC, ranging from 10.34% (2023) to 43.48% (2018). On the other hand, TN tumors represented 21.76% of the cases, with a range from 5.56% (2014) to 41.38% (2023) during the period, again with no significant change. VYWBC represented a steady 4% of NBC, with a non-significant tendency to decrease in recent years (reaching 1.84% in 2023). Stages II and III represented 38.51%, without significant changes throughout the period (p=0.437). Across subtypes, HR+ tumors were 47.19%, HER2+ 24.57% and TN, 27.03%. Interestingly, among YWBC, VYWBC represented a third of the new cases, peaking in 2020, followed by a numerical non-significant reduction in incidence (p=0.44). Conclusions: YWBC incidence has remained stable over the years. In our population, the YI is consistent with previous studies, ranging from 5-10% depending on the country, and slightly higher than the YI reported for Spain (6.6% in 2022). Both in YWBC and VYWBC, no variation was observed regarding stage and subtypes over time. Overall, these results indicate that over the past ten years there have not been significant changes in the YI, nor a trend towards more aggressive phenotypes among YWBC at our institution. Citation Format: Francisco Javier Muñoz i Carrillo, Benjamin Walbaum, María Rey, Carme Crous, Clara Rodrigo, Esther Sanfeliu, Fara Brasó-Maristany, Isabel Garcia-Fructuoso, Elia Seguí, Raquel Gómez-Bravo, Barbara Adamo, Tomás Pascual, Olga Martínez-Sáez, Francesco Schettini, Núria Chic, Montserrat Muñoz, Aleix Prat, Maria Vidal. Clinico-Pathological Characteristics and Incidence Trends of Breast Cancer in Young Women: A 10-Year Single Institution Retrospective Study [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2024; 2024 Dec 10-13; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2025;31(12 Suppl):Abstract nr P2-12-13.