Optimizing HER2/neu Detection in Equivocal (Score +2) Breast Cancer by Immunohistochemistry (IHC) via Fluorescence in Situ Hybridization (FISH): A Single-Center Experience.

Background & Objective: Breast cancer is a major global health concern, with HER2/neu overexpression observed in 15%-20% of cases, often indicating a more aggressive disease. It is important to accurately assess HER2 status using IHC and confirm it with FISH testing to guide targeted therapy decisions. The goal of this study was to improve HER2/neu detection in equivocal breast cancer cases, determine the rate of amplification in score +2 cases using FISH, and evaluate the correlation between FISH results and patient demographics such as age. Methods: This retrospective study analyzed 336 archived breast cancer cases with equivocal HER2/neu IHC scores (+2) from 2018 to 2024. IHC initially assessed HER2 expression, and ZytoLight® SPEC ERBB2/CEN 17 probes were used for FISH confirmation. Cases were classified into 5 ASCO-CAP 2023-defined groups based on HER2/CEP17 ratio and HER2 copy number. Final HER2 status guided clinical decision-making regarding eligibility for targeted therapy. Results: The majority of patients were female (91.1%) and aged 40-49 years (29.2%). HER2-negative status was most prevalent (71.1%), while 24.4% were HER2-positive. A significant association was found between gender and HER2 status (P = 0.002), with males showing higher HER2 positivity. A HER2/CEP17 ratio ≥2.0 was strongly associated with HER2 amplification (P = 0.0001).