HER2 Testing in Invasive Breast Cancer: A Comparison Between Immunohistochemistry and Fluorescence In Situ Hybridization Assays

HER2 status testing in breast cancer is crucial for the detection of eligible patients for trastuzumab therapy. In this study, the relative copy number of HER2 gene, in patients with breast cancer, was determined by fluorescence in situ hybridization (FISH) and the results were compared with those of immunohistochemistry (IHC) to obtain the concordance rate between these two methods.

HER2 status of 31 invasive breast cancer samples was compared using IHC and FISH techniques. The ratio of HER2/CEP17 was used to determine the amplification of the HER2 gene. If the ratio of HER2/CEP17 is greater than 2.2, HER2 gene amplification has occurred in the cancer cells. Then, a comparative analysis is performed to estimate the concordance rate between FISH and IHC results.

The gene amplification of HER2 was observed in 26% of cases by FISH. The IHC and FISH results showed 100%, 36.36%, and 85.71% concordance rates for cases with IHC scores of 3+, 2+, and 0/+1, respectively. The overall concordance between the two methods was 80%. Based on statistical analysis, HER2 status showed a considerable correlation with tumor grade (P= 0.02). No correlation was observed between HER2 gene status and the size and type of tumor, characteristics of lymph node, and patients’ age.

The data suggested that IHC results are reliable for HER2 status testing in cases with IHC scores 0/+1 and 3+. However, in patients with an IHC score of +2, it is necessary to perform a complimentary test to evaluate HER2 status to avoid haphazard treatment with trastuzumab in negative cases and identifying positive cases for suitable treatment.