Chromogenic in situ hybridization for Her‐2/neu‐oncogene in breast cancer: comparison of a new dual‐colour chromogenic in situ hybridization with immunohistochemistry and fluorescence in situ hybridization

Aims: Her‐2/neu testing is used as a marker for Herceptin^® therapy. The aim was to investigate new dual‐colour chromogenic in situ hybridization (CISH), in a large number of breast carcinomas (n = 205) with DNA‐specific dual‐colour probes (ZytoVision, Bremerhaven, Germany) and to compare the results with immunohistochemistry (n = 205) and fluorescence in situ hybridization (FISH) (n = 129). Methods and results: Paraffin‐embedded tissue of 205 patients was used. After immunohistochemistry with a focus on immunohistochemically uncertain cases, Her‐2/neu amplification using dual‐colour CISH (ZytoVision^®) was analysed. Validation by FISH was performed. The results were: immunohistochemistry, 27.8% with strong expression, 53.7% with uncertain overexpression and 18.5% with no expression; FISH, 25.6% amplified and 74.4% negative; CISH, 35.6% amplified, 62.9% negative and 1.5% not evaluable. Comparison of immunohistochemistry with CISH: CISH negative in 100% with immunohistochemistry 0/1+, amplified in 82.5% with immunohistochemistry 3+; 5.9% contradictory results: 4.4% immunohistochemistry 3+ and negative by CISH, 1.5% negative in immunohistochemistry but amplified by CISH; FISH (129 cases), 8.5% contradictory results to immunohistochemistry, 6.2% immunohistochemistry 3+ and negative by FISH, 2.3% negative by immunohistochemistry and amplified by FISH; comparison of CISH and FISH, 94.6% same results, 3.9% different ones, 1.6% CISH not analysable. Conclusions: CISH, using dual‐colour probes (ZytoVision^®) is as good as FISH for Her‐2/neu analysis. The few discrepant results are likely to be caused by polysomy or tumour heterogeneity.