Abstract PO1-27-11: Survival Benefit of Adjuvant Endocrine Therapy in Patients with Estrogen Receptor Low-Positive Breast Cancer

Abstract Background: Estrogen receptor (ER) low-positive (1–10%) breast cancer is a very rare subtype that accounts for only 2–5% of all breast cancer cases and was introduced as a new category in the 2020 American Society of Clinical Oncology/College of American Pathologist Guidelines. There are limited data on the overall benefit of endocrine therapy for patients with ER low-positive cancers. The objective of this study was to analyze the clinicopathological characteristics and treatment patterns in patients with ER low-positive breast cancer and to compare disease-free survival (DFS) between patients who received and did not receive endocrine therapy. Methods: A retrospective search of the prospectively maintained database of our institution identified consecutive women with ER low-positive (1–10%) primary invasive breast cancer who underwent curative surgery at Seoul National University Hospital (Seoul, Korea) between January 2010 and April 2021. Standard immunohistochemical staining techniques were used to assess the level of ER, progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) expression, and semiquantitative scores (0–100%) were reported for ER and PR. DFS was defined as the time from surgery to the development of any recurrence and was compared between women who received and did not receive adjuvant endocrine therapy using univariable and multivariable Cox proportional hazards regression. Results: A total of 721 women (median age, 51 years; range, 24–83 years) with ER low-positive cancer were included in the analysis. Overall, 28% (203/721) had AJCC stage I, 42% (304/721) had stage II, and 30% (214/721) had stage III. PR expression was negative (< 1%) in 79% (571/721), low-positive (1–10%) in 12% (90/721), and high-positive ( >10%) in 8% (60/721). HER2 expression was negative in 54% (390/721) and positive in 46% (331/721). Most women (88% [634/721]) received adjuvant endocrine therapy, including all women with PR expression >10%. Of the 661 women with PR expression ≤10%, 87 (13%) did not receive adjuvant endocrine therapy. HER2-targeted therapy was performed in 81% (268/331) of women with HER2-positive cancer. During a median follow-up of 5.7 years, 98 (14%) had recurrence (25 locoregional/contralateral and 73 distant metastases). AJCC stage III (HR, 3.6; 95% CI: 2.4, 5.4; P < .001) and receiving total mastectomy (HR, 1.8; 95% CI: 1.2, 2.8; P = .004) were associated with worse DFS. Receiving adjuvant endocrine therapy was independently associated with better DFS in the entire cohort (HR, 0.46; 95% CI: 0.28, 0.76; P = .002) and in the HER2-negative subgroup (HR, 0.40; 95% CI: 0.22, 0.75; P = .004). In the HER2-positive subgroup, receiving adjuvant endocrine therapy was not significantly associated with better DFS (HR, 0.55; 95% CI: 0.23, 1.32; P = .18). Conclusion: In this large cohort of women with ER low-positive breast cancers, adjuvant endocrine therapy was administered in the majority of cases, including those with PR >10% expression. Receiving adjuvant endocrine therapy was associated with better DFS, especially when HER2 expression was negative. Citation Format: Su Hyun Lee, Han Suk Ryu, Myoung-jin Jang, Kyung-Hun Lee, Han-Byoel Lee. Survival Benefit of Adjuvant Endocrine Therapy in Patients with Estrogen Receptor Low-Positive Breast Cancer [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO1-27-11.