Immunohistochemistry and FISH in HER2- positive breast cancer: Diagnostic techniques and clinical implications

HER2-positive breast cancer is a separate molecular subtype that bears significant implications for prognosis and therapy. The precise identification of HER2 status is paramount to the administration of targeted therapies, including trastuzumab and pertuzumab, with which the patients have gained substantial benefits. The two principal techniques for the assessment of HER2 status are Immunohistochemistry (IHC) and Fluorescence In Situ Hybridization (FISH). The IHC test measures HER2 overexpression, while FISH detects HER2 gene amplification; this provides complementary information on which to base the treatment decision. This review specifically discusses the underpinning principles, strengths and weaknesses, and clinical applications of HER2 testing via IHC and FISH methods, including their implications for treatment selection and prognosis. The article also discusses problems with accuracy of testing, variances in laboratory practices, and future technologies that may further allow refinement of HER2 testing. By integrating both IHC and FISH, the clinician can secure a more reliable diagnosis that leads to better defined therapeutic strategies and ultimately better outcome for their patients. Keywords: HER2-positive breast cancer, Immunohistochemistry (IHC), Fluorescence In Situ Hybridization (FISH), HER2 testing, targeted therapies, gene amplification, diagnostic techniques, clinical implications, breast cancer prognosis, personalized treatment