Abstract P2-09-05: Low estrogen receptor positive (1-10%) breast cancer prevalence, treatment patterns and prognosis: A single institution’s experience in Korea
Authors: Park Yeon Hee , Karantza Vassiliki , Calhoun Shawna R. , Park Seri , Lee Sohee , Kim Ji-Yeon , Yu Jong Han , Kim Seok Won , Lee Jeong Eon , Nam Seok Jin , Aktan Gursel , Marsico Mark 
Published: March 2020

Abstract
Background: Estrogen receptor (ER) expression is a predictive and prognostic factor in breast cancer. While the clinical benefit of endocrine therapy is indisputable, evidence suggests patients with low-ER expression (immunohistochemistry (IHC) 1-10%) may be less likely to respond to endocrine therapy and have a prognosis similar to those with hormone receptor negative breast cancer, particularly in the setting of human epidermal growth factor receptor 2 (HER2) negative disease. The objective of this analysis was to estimate the proportion of low-ER expression among female breast cancer patients, and the clinicopathological characteristics, treatment patterns and clinical outcomes using real world data.
Methods: A retrospective analysis was conducted using data from female breast cancer patients that underwent surgery with curative intent for invasive ductal or lobular breast cancer at Samsung Medical Center (Seoul, Korea) between January 2003 and December 2012. IHC was previously performed using standard procedures for determining ER, progesterone receptor (PR) and HER2 levels. PR expression was only used in determining triple-negative breast cancer (TNBC) status. ER-positive patients were further classified as ER expression low (1-10%) and high (&gt;10%) using their Allred Proportion Score (score=2) and Intensity Score (score of 1, 2 or 3). Disease-free survival (DFS) was analyzed using Kaplan-Meier method.
Results: The analysis included 5,930 patients with a median follow-up of 81 months. Overall, approximately 47% were stage 1, 41% stage 2, and 12% stage 3. Seventy-three percent of patients were ER+/HER2-, 13% TNBC, 8% ER+/HER2+ and 6% ER-/HER2+. The proportion of the cohort with low-ER expression was 2.0% (n=117); 63 (53.8%) and 54 (46.2%) with HER2- and HER2+ status, respectively. Among all ER+ women, 2.4% were classified as low-ER. The proportion of women with Ki-67 of ≥14 was greater in the low-ER/HER2- (90.0%) than the high-ER/HER2- cohort (43.3%) and similar to the TNBC cohort (92.2%). Low-ER/HER2- women were more likely to receive adjuvant chemotherapy (93.7%) than those with high-ER/HER2- (58%), which was comparable to women with TNBC (92.2%). Among those with HER2- status, low-ER patients were slightly less likely than their high-ER counterparts to have received adjuvant hormone therapy (92.1% vs 98.9%). Five-year DFS for the overall cohort was 91.7%; highest in the high-ER/HER2- cohort (94.0%), lowest in the TNBC cohort (81.3%) and 85.7% among low-ER/HER2- women.
Conclusion: Women with low-ER/HER2- breast cancer had similar clinicopathological characteristics, treatment patterns, and prognosis compared with those with TNBC. Clinical implication of this low-ER subgroup needs to be elucidated in terms of intrinsic subtyping based on biology to enable optimal treatment. Further research is needed to understand predictive and prognostic factors associated with low-ER/HER2- disease.
Citation Format: Yeon Hee Park, Vassiliki Karantza, Shawna R. Calhoun, Seri Park, Sohee Lee, Ji-Yeon Kim, Jong Han Yu, Seok Won Kim, Jeong Eon Lee, Seok Jin Nam, Gursel Aktan, Mark Marsico. Low estrogen receptor positive (1-10%) breast cancer prevalence, treatment patterns and prognosis: A single institution’s experience in Korea [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P2-09-05.