Abstract PO2-17-09: Overall survival differences between HER2 Low and other breast cancer subtypes in a reference center in Brazil

Abstract Background: Breast cancer (BC) is a heterogeneous disease, and its subtypes have different prognosis and diverse responses to endocrine therapy and chemotherapy. In clinical practice, immunohistochemical markers are often used to classify BC into subtypes according to the presence of hormone receptors, human epidermal growth factor receptor 2 (HER2) and evaluation of proliferation measured by Ki67. Recently, there has been a suggestion for a possible alternative terminology for situations involving IHC 1+ or 2+ with negative ISH, this proposed term is referred to as HER2-low and represents about 50 to 55% of all primary BC. There are relevant gaps regarding the outcomes of HER2-low BC. Considering the new emerging therapies for HER2-low, a better description of the epidemiology, response to treatment and results of this population is extremely relevant. Objective: The purpose of this study was to use real-world data in order to investigate the Brazilian patients with BC treated in a Reference Center and evaluate the survival outcomes in different subtypes identified by molecular immunohistochemistry including HER2-Low and evaluate the survival in subtypes by stage. Methods: This retrospective cohort study included women, age 18 years with breast cancer treated between 2010 and 2019 at Perola Byington Hospital in the city of São Paulo, Brazil. After the diagnose, the included patients were submitted to an immunohistochemical analysis to identify the subtype of the BC. Institutional Review Board of Pérola Byington Hospital approved the use of patients' data before the beginning of the study, under the reference number CAAE 7238317.6.0000.0069). An informed consent was dismissed, and this study was carried out according to the Declaration of Helsinki, including protection of patients' confidentiality. The primary outcome measure was overall survival (OS), which was defined as the length of time (in months) from the date of diagnosis to the date of death (from any cause). The secondary outcome measure was disease-free survival (DFS), which was defined as the length of time after the end of the primary cancer treatment in which the patient survives without any signs or symptoms of cancer. Continuous variables were summarized using their mean. Qualitative variables were summarized for the general population and by HCI subgroups using counts, percentages, and absolute numbers. The Kaplan-Meier method was used to estimate survival rates and mean survival times in the general population and by groups. All statistical tests were bilateral and p 0.05 was considered significant. Statistical analyses were performed using the Python programming language version 3.8. Results: Between January 2010 and December 2019, 1,826 women were diagnosed with breast cancer and classified as a HER2-low subtype, among the 9,278 participants included in the study (19,68%). Regarding the OS analysis, patients with the "Luminal A" subtype had better survival (106 months) results when compared with the other subtypes and women with triple negative disease had the worse prognosis with OS of 96 months. The median survival in HER2-low with hormonal receptor positive was 101 months and statistically significant when compared with 90 months in HER2-low hormonal receptor negative (p 0,01). When looking for stage IV HER2-low and HER2+ have better overall survival rates compared to other subtypes. Conclusion: This real-world data study shows difference in the prevalence of the HER2-low subtype. In addition, there is a difference in OS between tumor subtypes and when evaluating the HER2-low subtype with or without the presence of receptors, this difference is marked. Citation Format: Andre Mattar, Andressa Amorim, Marina Diogenes, Marcelo Antonini, Francisco Pimentel Cavalcante, Luiz Henrique Gebrim. Overall survival differences between HER2 Low and other breast cancer subtypes in a reference center in Brazil 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 PO2-17-09.