Abstract Background: Over the past two decades, breast cancer characteristics and outcomes have improved, largely due to earlier detection through increased awareness and screening, as well as improvements in (neo-)adjuvant therapies. However, real-world data on breast cancer in women up to age 40 remain rare. Methods: This retrospective study compares demographics, tumor characteristics, therapies and 5-year survival between two cohorts of female patients with invasive breast cancer diagnosed before the age of 41 at UZ Leuven, during the periods 2000-2005 (cohort 1) and 2014-2019 (cohort 2). The Mann-Whitney U test was used for continuous variables and Fisher’s exact test for categoric variables. Significance was defined if p0.05. Results: Cohort 1 consist of 233 patients, with a mean age of 35.5 years at diagnosis. Cohort 2 consists of 237 patients, with a mean age of 34.9 years at diagnosis. A germline mutation was detected in 36/108 (33.3%) of patients in cohort 1 and in 51/206 (24.8%) of patients in cohort 2; this did not significantly differ between the two cohorts. See table: At diagnosis, patients in cohort 2 were significantly more likely to have been detected through screening, to have a second-degree relative with breast cancer, to be pregnant, to have had their first full-term pregnancy at an older age, to have breastfed and to have used an intrauterine device. They were significantly less likely to have used oral contraceptives at diagnosis. In addition, tumors were significantly more likely multifocal and ER1% HER2 amplified, but significantly less likely lobular, grade 3, and ER 1% HER2 amplified. Significant more patients in cohort 2 were metastatic at diagnosis, received neoadjuvant and anti-HER2 therapy. At year 5, significantly more patients in cohort 2 were disease free, distant relapse free and alive. Conclusion: A comparison of women under 41 years diagnosed with invasive breast cancer across two time periods revealed differences in demographics, tumor characteristics, systemic treatments, and substantial disparities in outcomes. Mean age at first full-term pregnancy significantly increased with more than 1 year, the number of multifocal tumors doubled, the number of lobular tumor decreased from nearly 8% to nearly 1%, the number of patients that received neoadjuvant therapy nearly tripled and 5-year distance relapse decreased from nearly 20% to nearly 6% when cohort 1 was compared to cohort 2. Citation Format: B. Goots, A. Laenen, S. Han, M. Van Houdt, Y. Van Herck, F. Derouane, C. Weltens, H. Janssen, A. Baten, J. Verhoeven, G. Floris, C. Desmedt, K. Van Baelen, A. Smeets, I. Nevelsteen, C. Van Ongeval, M. Keupers, V. Celis, A. Coessens, R. Prevos, H. De Boodt, H. Wildiers, P. Neven. Two cohorts of young women (41y at diagnosis) with invasive breast cancer: Single center data on how demographics, tumor characteristics, therapies and 5-year survival differ when treated 2000-2005 and 2014-2019 abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS2-06-16.
Goots et al. (Tue,) studied this question.