Abstract PS3-09-05: Real-world clinical characteristics and treatment patterns among UK and French patients with a recent diagnosis (2020 onwards) of early-stage, estrogen receptor-positive, human epidermal growth factor receptor 2-negative breast cancer (ER+/HER2-eBC)

Abstract Background: ER+/HER2-eBC is heterogeneous, with a subset of patients at high risk of recurrence and poorer outcomes. The treatment landscape for the high-risk group has shifted recently, with the addition of targeted therapies to clinical guidelines. This study aimed to describe the clinical characteristics and treatment patterns for recently diagnosed high-risk patients. Methods: This was an observational, retrospective cohort study using electronic health records from five oncology centers across France and the UK. The study included patients with recently diagnosed (2020-2024), high-risk (Grade III and T1c-T2/N1-N2 or T3-T4/N0-N2), ER+/HER2- eBC, with a minimum follow up of 12 months. Initial data on baseline characteristics and real-world treatment patterns (surgery, chemotherapy [CT], radiotherapy [RT] and other treatments) from first diagnosis to the time of appearance of metastasis are presented. Results: The study included 421 patients (68% between 35 and 65 years old); 239 (57%) from France and 182 (43%) from the UK. Of the 267 patients with known status, 164 (61%) were post-menopausal women. Patients were diagnosed with T1c-T2 and N1-N2 (309, 73%) or T3-T4 and N0-N2 (112, 27%) BC (including American Joint Committee on Cancer, version 7 stages IB, II, IIIA, IIIB). At baseline, 357 (85%) were ER-positive (≥10% expression). The majority were lymph node positive (369, 88%): 307 (73%) had 1-2 positive lymph nodes and 62 (15%) had ≥3. BRCA testing at diagnosis was performed in 104 (25%) patients with 17 (16%) positive. As expected, prognostic genomic testing was rarely conducted in this population. Of 277 patients with ECOG available at the start of treatment, 223 (81%) had a score of 0. Almost all patients (413, 98%) underwent eBC surgery and, among these, lumpectomies alone (191, 45%) and mastectomies alone (155, 37%) were the most common surgeries. Of the 421 patients included, many received adjuvant CT (204, 48%) and RT (303, 72%) post-surgery. Neoadjuvant CT was received by 178 patients (42%) and RT by 5 patients (1%). Based on initial findings, adjuvant use of abemaciclib increased from 3/66 patients (5%) diagnosed in 2020 to 51/108 patients (47%) in 2023 ( Table ). Conclusions: These results showcase recent real world clinical practice in France and the UK for patients with high-risk ER+/HER2- eBC. Most patients received surgery with (neo)adjuvant CT and other treatments; specifically, there was a substantial increase in the use of adjuvant abemaciclib during the study period. Detailed insights on specific treatment agents, regimen sequences received, and breakdowns by country will be provided in the symposium publication. Citation Format: P. Cottu, E. Sawyer, J. Frenel, J. R. Earla, K. M. Hirshfield, G. Gooud, D. Ntais, A. Jamotte, C. Perkins, A. Babonneau, E. Kitetere, M. Vallet, M. Riaz, J. Timbres, L. Haroun, L. Cirneanu, R. Hermans, V. Saglimbene, K. Dushkin, A. Ajmal, S. Oikonomou, A. Vladimirova, K. Desai, F. Bocquet, P. S. Hall. Real-world clinical characteristics and treatment patterns among UK and French patients with a recent diagnosis (2020 onwards) of early-stage, estrogen receptor-positive, human epidermal growth factor receptor 2-negative breast cancer (ER+/HER2-eBC) [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 PS3-09-05.