High accuracy in detecting HER2‐low status in FNA of primary and metastatic breast cancer

HER2‐positive invasive breast carcinomas (IBCs) account for 15% of breast cancers and are driven by ERBB2 gene amplification. Although historically associated with aggressive behavior, HER2‐targeted therapies have significantly improved outcomes. HER2 status is routinely assessed by immunohistochemistry (IHC) and in situ hybridization (ISH). Recently, tumors with low HER2 expression (IHC 1+ or 2+ without amplification), previously classified as HER2‐negative, have emerged as a clinically relevant subgroup. Fine‐needle aspiration cytology (FNAC) is a minimally invasive alternative to core needle biopsy, particularly useful in inoperable or metastatic settings. FNAC‐derived cell blocks (CBs) allow immunocytochemical (ICC) evaluation of biomarkers.