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Comparison between immunohistochemistry and chromogenic in situ hybridization in assessing HER‐2 status in breast cancer

Human epidermal growth factor receptor‐2 (HER‐2) is usually determined as a potential target for breast cancer therapy. The purpose of the present study was to compare chromogenic in situ hybridization (CISH) with immunohistochemistry (IHC) in determination of HER‐2 status, in metastatic breast cancer patients screened for the clinical study of chemotherapy ± herceptin. It was possible to assess both CISH and IHC in 56 cases, using CISH Detection Kit (Zymed) and HercepTest (DakoCytomation), respectively. HER‐2 was amplified by CISH in 32 cases (57%) while 33 (59%) were HER‐2‐positive by IHC. A concordance between HER‐2 status determined by CISH and IHC was noted in 43 of 56 cases (77%; P = 0.00008). Gene amplification was observed in 6/16 cases (37.5%) in IHC‐negative subgroup (1+), while no amplification was observed in 5/10 cases (50%) in the IHC‐positive subgroup (2+). These results suggest that there was a greater heterogeneity on the genetic level and that simple IHC classification was not sufficient. It is suggested that CISH could be considered as a useful additional method to IHC in determining HER‐2 status in breast cancer patients, with a recommendation for testing not only the 2+ but also the 1+ subgroup of patients.