HER2 positivity in breast carcinoma: a comparison of chromogenic in situ hybridization with fluorescence in situ hybridization in tissue microarrays, with targeted evaluation of intratumoral heterogeneity by in situ hybridization.

An accurate and reproducible assay method for determining HER2 status is crucial, as a positive HER2 gene status is an eligibility requirement for Herceptintrade mark therapy. Although immunohistochemical (IHC) assessment is both practical and inexpensive, a worrying trend of high false-positive rates has been reported. Fluorescence in situ hybridization (FISH) is the universally accepted gold standard for confirming IHC 2+ cases and ambiguous results but is costly and requires specialized equipment and technical expertise. Chromogenic in situ hybridization (CISH) amalgamates the practical advantages of IHC with the reproducibility of FISH, and high concordance between the CISH and FISH methods has been reported in conventional sections. Tissue microarrays (TMAs) allow high throughput of specimens, and HER2 status assessment in TMA cores using IHC and FISH has correlated well with scores in conventional sections. The authors used TMA technology to compare the efficacy of ZYMED(R) CISH with PathVysiontrade mark FISH in a cohort of 119 archival breast resection cases and investigated possible intratumoral heterogeneity in a "mini-array" of 21 HercepTest "equivocal"/2+ cases. Concordance between FISH and CISH in TMA sections was 99%. All prescored 2+ HercepTest cases were nonamplified. Four 3+ HercepTest cases were classed as potential false-positives. The authors suggest that confirmatory ISH should be performed on all positive HercepTest cases. CISH was easier to perform and quicker to enumerate than FISH. The authors conclude that CISH is a practical alternative to FISH as a confirmatory tool for HER2 gene amplification status. Intratumoral heterogeneity did not affect the patient's HER2 status.