Abstract PS3-07-06: Real-world Prognostic Evaluation of Adjuvant Endocrine Therapy, Ovarian Function Suppression, and Chemotherapy in Premenopausal HR-positive/HER2-negative Early Breast Cancer in Japan: The PEACE Study

Abstract Background: Premenopausal women with hormone receptor-positive, HER2-negative early breast cancer (HR+HER2- EBC) exhibit higher incidence rates in Asia than in Western populations. Chemotherapy (CT) is routinely recommended for premenopausal node-positive patients, while it remains unclear whether its apparent benefit derives primarily from ovarian function suppression (OFS). We therefore evaluated long-term outcomes of adjuvant endocrine therapy (ET) alone, ET plus OFS, and ET plus chemotherapy (±OFS) in a nationwide Japanese cohort, stratified by nodal status. Methods: We retrospectively analyzed 7,396 premenopausal HR+HER2- T1-4N0-1 patients who underwent surgery at 25 Japanese centers between January 2008 and December 2017. Patients were divided into node-positive (N1; n=2,041) and node-negative (N0; n=5,355) cohorts. Three treatment groups were defined: ET alone, ET+OFS, and ET +chemotherapy (±OFS). Survival probabilities of eight-year disease-free survival (DFS) and overall survival (OS) were estimated by Kaplan-Meier methods; hazard ratios (HRs) for treatment groups were estimated using Cox models with inverse probability of treatment weighting (IPTW) adjusted for age, tumor size, grade, receptor status, lymphovascular invasion, systemic treatment pattern, surgery type, and radiotherapy. Results: Median follow-up was 8.4 years. In the overall population (n=7,396), 8-year DFS and OS were 95.2% and 98.0%, respectively. Among N0 cohort (n = 5,355), 68.4% ≦T1 and 31.6% ≧T2. Histological grade 3 comprised 12.7%. ER positive in 82.3%; PgR positive in 75.1%. N0 patients’ 8-year DFS was 97.1% (95% CI 96.6-97.6), and 8-year OS 98.8% (95% CI 98.4-99.2), with no significant DFS benefit for ET+OFS or CT versus ET alone. Among N1 cohort, 42.1% ≦T1 and 58.8% ≧T2. Histological grade 3 comprised 28.5%. N1 patients’ 8-year DFS was 90.4% (95% CI 88.7-92.1), and 8-year OS 96.0% (95% CI 94.9-97.1). Compared with ET alone, ET+OFS conferred a significant DFS reduction, whereas CT did not reach significance. OS differences were non-significant in both subgroups. In a Cox model for DFS in N1 patients, the following factors were independently associated with statistically favorable DFS; ET+OFS treatment pattern (vs ET alone), PgR positive (vs PgR null). ET+Chemotherapy was favorable DFS trend with ET alone. Conclusions: Japanese premenopausal HR+HER2- EBC patients demonstrate excellent long-term survival, comparable to Western clinical-trial populations. In node-positive disease, OFS plus ET provided numerically greater DFS benefit than chemotherapy, suggesting that OFS may allow omission of chemotherapy in selected N1 patients. Prospective validation, is warranted to optimize adjuvant treatment in this population. Citation Format: T. Shimoi, K. Narui, K. Kataoka, S. Orihara , K. Kida , R. Nakamura , K. Adachi, Y. Horimoto, M. Oshi, A. Yamada, K. Matsumoto, J. Tsurutani, Y. Kajiura, H. Nogi, S. Akashi-Tanaka, Y. Hasegawa, K. Wakita, T. Kubota, M. Taguri, T. Ishikawa. Real-world Prognostic Evaluation of Adjuvant Endocrine Therapy, Ovarian Function Suppression, and Chemotherapy in Premenopausal HR-positive/HER2-negative Early Breast Cancer in Japan: The PEACE Study [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 PS3-07-06.