Determination of HER2 gene amplification by fluorescence in situ hybridization and concordance with the clinical trials immunohistochemical assay in women with metastatic breast cancer evaluated for treatment with trastuzumab

Purpose. To evaluate the concordance between HER2 gene amplification, determined by fluorescence in situ hybridization (FISH), and HER2 protein overexpression assessed by an immunohistochemical (IHC) assay. The IHC protocol used was a research assay, known as the Clinical Trial Assay (CTA), developed to select women with metastatic breast cancer (MBC) for three pivotal clinical trials of trastuzumab therapy. Methods. A direct-labeled, dual-probe FISH assay was used to determine HER2 amplification in 623 fixed breast cancer tissue specimens. These specimens had been stored as paraffin-embedded sections for 2-5 years. All specimens had been analyzed for HER2 protein expression by the CTA. To assess the reproducibility of FISH results in archived material, we evaluated a separate group of 617 breast cancer tissue specimens at two different laboratories. Results. Informative FISH results were available for 529 (85%) of the 623 specimens. Overall concordance between FISH and IHC results was 82% (95% CI; 78-85%). Assay agreement between FISH results and specimens with immunostaining scores of 0, 1+, and 3+ were 97, 93 and 89%, respectively. However, only 24% of specimens with 2+ immunostaining scores had HER2 amplification by FISH; there was assay disagreement in 76% of specimens in this IHC subgroup. Interlaboratory FISH concordance was 92% (95% CI; 89-94%), indicating very good assay reproducibility in these archived specimens. Conclusion. HER2 status determined by CTA-IHC and FISH are significantly correlated; however, differences between these two assays can affect patient selection for trastuzumab therapy.