Abstract PO4-25-11: Inconclusive Her2 by immunohistochemistry compared to in situ hybridization in breast cancer: a cross-sectional study

Abstract

Abstract Introduction: Breast cancer stands as the most common malignancy diagnosed among women worldwide, with the Her2-positive subtype accounting for approximately 15% of the cases. The introduction of HER-targeted therapies has changed the natural history of this subtype, however, evaluating the Her2 status using immunohistochemistry (IHC), can yield inconclusive results in a significant percentage of cases, which requires the use of in situ hybridization techniques. The study of such cases has not been thoroughly explored, particularly in the brazilian population, and increasing interest in this subgroup of patients arises as the "Her2 low" category gathers evidence of clinical applicability. The present study aimed to assess the final Her2 status of samples with indeterminate IHC results and compare these findings with current literature. Methods: We conducted a cross-sectional study at the breast unit service of a private health network in the state of Minas Gerais, Brazil (Rede Mater Dei de Saúde). Breast tissue samples obtained over the past 5 years were analyzed, and specimens with inconclusive IHC results for Her2 were included in the study. Subsequently, the final Her2 status was determined through fluorescence in situ hybridization (FISH) test, or reanalysis in cases in which a new sample (surgical specimen) was obtained. Results: Among the initially identified 73 samples with indeterminate IHC results for Her2, a total of 15 patients were excluded due to lack of follow-up within the breast unit service. The final cohort consisted of 58 women aged between 28 and 71 years. Notably, approximately 91% of the patients exhibited positive hormone receptor tumors, whereas 9% displayed negative hormone expression. After reviewing the medical records, approximately 36% of the patients were classified as having a positive final Her2 status, while 62% had a negative final status. One patient had an inconclusive FISH result. Conclusion: The present study revealed that approximately 10% of the breast tissue samples included in the study yielded inconclusive results for Her2 by immunohistochemistry. Following FISH testing, the majority of these patients were reclassified into the negative final Her2 status category. The classification of most patients to what is known as the emerging "Her2 low" category, aligns with international publications, and highlights the clinical relevance of exploring this subset of breast cancer that gains relevance as advances provide prospects for targeted therapies. Table 1. Summarized results for the final Her2 status Absolute and percentage values for the final HER2 status in the study cohort Citation Format: Felipe Marcondes de Oliveira Coelho, Renata Capanema Saliba Franco, Enaldo Melo de Lima, Marco Antonio Dias, Anna Dias Salvador, Waldeir José de Almeida Junior, Julia Wanderley Rennó, José Tadeu Campos de Avelar. Inconclusive Her2 by immunohistochemistry compared to in situ hybridization in breast cancer: a cross-sectional study [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 PO4-25-11.