Abstract INTRODUCTION: Endocrine prevention is a well-established preventative strategy for women with high-risk lesions (HRL). The objective of the study was to evaluate trends and factors associated with endocrine prevention adherence in this patient population. METHODS: We performed a retrospective cohort study of all women referred for high-risk evaluation due to diagnosed atypical ductal hyperplasia (ADH), atypical lobular hyperplasia (ALH), and lobular carcinoma in situ (LCIS) between 2019-2025. Data pertaining to eligibility, initiation, and adherence to endocrine prevention were extracted from the medical record. Univariate analyses were performed using Chi-squared and Fisher’s exact test to evaluate factors associated with adherence. Results: Of 216 female HRL patients 188 met criteria and were included in the analytic cohort including 107 (56.9%) with ADH, 66 (35.1%) with ALH, and 15 (8%) with LCIS. The median age was 55 years (interquartile range IQR: 50-62). Overall, 106 (56.4%) HRL patients accepted a prescription of endocrine prevention. Low-dose tamoxifen was the most common regimen prescribed (64.2%), compared to regular dose tamoxifen (11.3%), raloxifene (19.8%), and anastrozole (4.7%). At a median follow up of 28 months (IQR 11-46 months), 71 women (67% of those who accepted a prescription) initiated endocrine prevention and 65 (61.3%) remained adherent. Lack of prior hormone replacement therapy use (65.6% vs 30.8%, p=0.02) and family history of breast cancer (77.8% vs 55.7%, p=0.04) were significantly associated with adherence to endocrine prevention. Adherence was not significantly different by regimen prescribed (p=0.66), nor was it significantly associated with age (p=0.08) or prior surgical excision of their HRL (p=0.52). Of the 42 patients who initiated tamoxifen 5 mg, 39 (92.9%) remained adherent during follow up, and of 12 women (23%) who had completed at least 3 years of therapy, 10 (83.3%) elected to continue or complete a total 5-year course of low dose tamoxifen. Conclusion: Adherence to endocrine prevention is high in women who initiate medication with few discontinuing due to side effects. Many women who initiate low dose tamoxifen remain adherent, with some electing to continue the regimen for 5 years total. Citation Format: N. Stonebanks Cuillerier, H. Almarzooqi, V. Villareal-Corpuz, S. Saboobheh, I. Prakash, M. Basik, J. Boileau, K. Martel, S. Meterissian, M. Pollak, S. M. Wong. Adherence to endocrine prevention in patients with atypical hyperplasia and lobular carcinoma in situ: promising trends from real-world use of low dose tamoxifen. 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 PS2-04-10.
Cuillerier et al. (Tue,) studied this question.