Abstract PS3-10-19: Prevalence of HER2-low status in Ecuadorian women with Hormone Receptor Positive Breast Cancer

Abstract BACKGROUND: Breast cancer (BC) is the most frequent neoplasia in women worldwide. According to GLOBOCAN(2022), 9.6% of new cases globally occur in the Caribbean and Latin America. The prevalence ofHER2-low status in South America and particularly in Ecuador is limited. Although many HER2-low tumors coexpress hormone receptors (HR), their exact global prevalence remainsundetermined. Therefore, this study aims to determine the prevalence of HER2-low status inEcuadorian women with HR-positive BC. Methods: A retrospective, cross-sectional study of BC patients' medical records (2011-2021) fromEcuador's SOLCA oncological hospital. Simple random sampling included 508 patients (p). HR-positive patients express estrogen receptors; HR-negative do not. HER2-low status is defined asHER2 1+ or HER2 2+/SISH- (silver in situ hybridization). Univariate analyses measuredfrequencies and percentages. Result: sAmong the 723p evaluated, 215 were omitted from the analysis owing to missing data on HER2status. Of 508p with evaluable HER2 expression, 144 (28.3%) were HR-negative and 364 (71.7%)HR-positive. Of the 364p HR-positive, 135 (26.6%) presented with HER2 0+, 66 (13.0%) with HER21+, 64 (12.6%) with HER2 2+ (42 SISH-, 11 SISH+ and 11 SISH not available), and 99 (19.5%) withHER2 3+. Therefore, 108p (21.3%) had tumors classified as HER2-low and HR-positive. Clinicalstage II was the most prevalent among HER2-low HR-positive patients. Conclusions: The prevalence of HER2-low status in Ecuadorian women with HR-positive BC was 21.3%. AmongHR-positive patients, 108 of 364 (29.7%) exhibited HER2-low status, which is comparable to the34% reported in a Thai cohort of HR-positive cases. Conversely, among HER2-low patients, 108of 132 (81.8%) were HR-positive, exceeding the 71.1% reported in a ten-country study spanningEurope, Asia, and North America. Table 1 Baseline characteristics of patients n=508; results in frequency and (%) Age years, mean (s.d.)54.4 (12.7) Initial Clinical Stage I59 (11.6) II241 (47.4) III157 (30.9) IV22 (4.3) Progesterone receptors Positive345 (67.9) Negative163 (32.1) Estrogen receptors (ER) Positive364 (71.7) Negative144 (28.3) Ki-67 < 14100 (19.7) ≥ 14403 (79.3) HER2 0+172 (33.9) 1+84 (16.5) 2+ SISH-48 (9.4) 2+ SISH+13 (2.6) 2+ SISH (NA)13 (2.6) 3+178 (35.0) HER2-low y ER+108 (21.3)s.d.: standard deviation, SISH: silver in situ hybridization, ER: estrogen receptor, NA: not available Citation Format: L. Estrada, W. Morquecho, V. Bastidas, R. Escala. Prevalence of HER2-low status in Ecuadorian women with Hormone Receptor Positive Breast Cancer [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 PS3-10-19.