Most tests developed for the determination of the HER2 status still require validation, although identification of the HER2 status is important for predicting the response to specific systemic therapy in breast cancer. Fluorescence in situ hybridization (FISH) and immunohistochemistry (IHC) were performed for the HER2 expression on the tissue microarray (TMA) from primary breast cancers to validate the feasibility of IHC for HER2 assay.

A TMA was constructed from 134~231 primary breast cancers. FISH and IHC were repeated more than twice, and the results were analyzed. Three kinds of primary antibody for IHC were used and compared.

The HER2 was amplified by FISH in 24~28% of breast cancer with a concordance between multiple assays of 92~100% (kappa=0.994-0.965), while the HER2 was overexpressed in 21~27% by IHC. The HER2 was amplified in 70~100% of the IHC 3+ cases, but was observed in only 45~78% and 5~12% of the IHC 2+ and IHC 0~1+ cases, respectively. The results from the IHC, using 3 different primary antibodies to HER2, were in good agreement each other at 88~92% (kappa=0.902-0.799).

The results of the FISH appeared to be more reproducible than those of the IHC in the current study. The results of the IHC were not different from each other according to primary antibody used. However, a considerable proportions of the IHC positive cases were not confirmed by the FISH. This report indicates a need to improve the laboratory quality control measures when using the IHC for the HER2 assay, including the periodic testing for the concordance with FISH.