Abstract PO4-03-10: HER2 Expression in Triple-Negative Breast Cancer: Clinicopathologic Features and Response to Neoadjuvant Chemotherapy

Abstract BACKGROUND: Half of the primary breast cancers reported as HER-2 negative have low HER-2 expression, defined as an immunohistochemical (IHC) score of 1+ or 2+, with negative in situ hybridization. Among these cases, 37% correspond to the triple-negative immunophenotype. The primary objective of this study was to evaluate HER-2 expression in patients with triple-negative breast cancer and compare the clinicopathologic features and response to neoadjuvant chemotherapy between HER-2 negative (0) and HER-2 low (1+/2+) subtypes. METHODS: We retrospectively analyzed 157 patients with early-stage triple-negative breast cancer who underwent neoadjuvant chemotherapy followed by surgery with curative intent and met the study's inclusion criteria. These patients were treated at the National Institute of Neoplastic Diseases between 2010 and 2021. HER-2 analysis was performed using immunohistochemistry (IHC), and in cases of HER-2 (2+) amplification, in situ hybridization was performed. RESULTS: Of all the included patients, 107 were classified in the HER-2 negative group (68%), and 50 were classified in the HER-2 low group (32%). Seventy-four percent of patients with HER-2 negative were in the age range of 40-64 years, compared to 68% in the HER-2 low group. A significant difference was found between the HER2-negative and HER2-low groups regarding age and Ki67. The median age of HER-2 low patients is significantly higher than the median age of HER-2 negative patients (52 years vs. 45 years, respectively). No significant association was found between HER-2 status and clinical stages or histological grade. However, the median Ki67 percentage of HER-2 negative patients is significantly higher than the median Ki67 of HER-2 low patients (70% vs. 50%, respectively (p=0.035)). Thirty-two percent of patients with HER-2 negative achieved a pathological complete response (pCR) compared to 18% of patients with HER-2 low. There were no differences in the pCR rate in the HER-2 1+ and 2+ subgroups. Among the patients included in both groups, who achieved pCR, no disease progression was observed at the time of analysis. CONCLUSIONS: Patients in the HER-2 negative group had a higher rate of complete pathological response to neoadjuvant chemotherapy compared to those in the HER-2 low group. No differences were observed in the pCR rate between HER-2 1+ and 2+ subgroups, suggesting that HER2 expression levels within the low range may not significantly impact treatment response. Table. Table. Citation Format: Zaida Morante, Iris Otoya, Natalia Valdivieso, Cindy Calle, Hugo Fuentes, Silvia Neciosup, Carlos Castañeda, Tatiana Vidaurre, Martin Falla, Valeria Colomo, Henry Gómez. HER2 Expression in Triple-Negative Breast Cancer: Clinicopathologic Features and Response to Neoadjuvant Chemotherapy 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 PO4-03-10.