Abstract INTRODUCTION: Women diagnosed with atypical hyperplasia (AH) and lobular carcinoma in situ (LCIS) are at higher risk of developing invasive breast cancer. This risk may be reduced by endocrine prevention, though patient uptake and compliance are low. We sought to evaluate factors associated with the uptake and initiation of endocrine prevention therapy among women with AH and LCIS within a real-world setting. Methods: This retrospective cohort study included women with high-risk lesions (HRL) diagnosed on biopsies performed between 2014 and 2025 and referred to the Jewish Stroll Cancer Prevention Centre and Stroll Cancer Prevention Centre, including women with AH and classical LCIS. The primary outcome was uptake of endocrine prevention, defined as receipt of a prescription and endocrine prevention initiation in eligible women. Results: Endocrine prevention was prescribed to 106 (56.4%) of 188 eligible patients, and 71 (67.0%) started treatment. The regimen most frequently prescribed was low-dose tamoxifen at 64.2%, Raloxifene at 19.8%, and standard-dose tamoxifen at 11.3%. The highest uptake was 73.4% for women aged 51-60 years compared to 56.5% of women aged 41- 50 years, and 41.5% of women aged above 60 years (p 0.001). A high uptake rate was also observed in parous women (64.6% vs 44.0%, p =0.005) and in women with a history of oral contraceptive use (69.5% vs. 46.2%, p = 0.001). Before 2019, prescription rates were similar between Raloxifene and standard dose tamoxifen (47.1% each). However, low-dose tamoxifen became the most commonly prescribed regimen after 2019 (76.4%, p 0.001). Conclusion: After 2019, low-dose tamoxifen became the most commonly prescribed endocrine prevention regimen in HRL patients who accepted a prescription. Citation Format: H. Almarzooqi, N. Cuillerier, S. Wong, V. Villareal-Corpuz, M. Basik. Factors associated with uptake of endocrine prevention in patients with atypical hyperplasia and lobular carcinoma in situ 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 PS3-03-02.
Almarzooqi et al. (Tue,) studied this question.