Comparison of chromogenic <Emphasis Type="Italic">in situ</Emphasis> hybridization with other methodologies for HER2 status assesment in breast cancer

The high incidence of human epidermal growth factor receptor (HER)2 overexpression on breast and various other cancer cells and the prognostic and potentially predictive value of HER2 render this growth receptor a novel and important therapeutic target. Out of a wide range of assays that have been used in research for the detection of HER2 status, only two techniques are now predominant and readily applicable in the routine clinical pathology laboratory: determination of HER2 overexpression by immunohistochemistry (IHC) and HER2 gene amplification by fluorescence in situ hybridisation (FISH). In a retrospective study on a cohort of 173 archival invasive breast carcinomas a chromogenic in situ hybridisation (CISH) assay for the detection of HER2 amplification was established. Results were compared to HercepTest, which is the most frequently used method for detecting HER2 alteration. Additionally, HER2 gene copy number was investigated using differential PCR (dPCR) as a testing system. Discrepant cases between CISH and HercepTest and all IHC positive cases (2+ and 3+), a total of 42 cases, were analysed with FISH Path Vysion (Vysis) assay. HER2 overexpression was found by IHC in 24.3%, HER2 amplification by CISH in 19.1% and by dPCR in 9.2% of the tumours. The overall concordance rate between CISH and IHC was 95.9%, between dPCR and IHC 85% and between CISH and FISH 100%, respectively. Among 25 HercepTest positive cases (score 3+) two showed no gene amplification and four out of 13 tumours with score 2+ were negative with CISH and FISH. The current study showed that CISH offers an ideal approach that allows detection of HER2 amplification in the context of morphology, whereas the major drawback of dPCR is the impracticability of tissue differentiation of invasive and non-invasive carcinoma.