Determination of HER2/neu status: A pilot study comparing HER2/neu dual in situ hybridization DNA probe cocktail assay performed on cell blocks to immunohistochemisty and fluorescence in situ hybridization performed on histologic specimens

• Context.-Validation of new methodologies for determining human epidermal growth factor receptor 2 gene (HER2/neu) amplification status is crucial for advancing the standard of care and determining treatment for patients with primary and/or metastatic breast carcinoma. Objective.-To compare results of HER2/neu gene amplification status by 2-color chromogenic in situ hybridization (ISH) on cell block material to HER2/neu status by immunohistochemistry (IHC) and/or fluorescence in situ hybridization (FISH) in the corresponding resection specimen or previous biopsy specimen. Design.-Formalin, thrombin, and Cellient cell blocks were prepared from cytologic samples obtained from resection specimens from 27 patients with invasive breast carcinoma. In situ hybridization was performed on cell block sections from 18 of the collected cases, on both the Ventana BenchMark ULTRA and the Ventana BenchMark XT, and the HER2/neu gene amplification status was determined. This was then compared to the HER2/neu status by IHC and/or FISH in the resection specimen or previous biopsy specimen. Results.-Comparison of HER2/neu status by ISH on the quantifiable cell block sections showed 100% correlation with the HER2/neu status determined by IHC or FISH in the corresponding histologic specimens. The results from thrombin and formalin cell blocks were statistically superior to the results from Cellient cell blocks on both Ventana instruments. Conclusions.-While further validation and study are needed, preliminary results show that the HER2/neu gene amplification status of breast carcinomas can reliably be determined on thrombin and formalin cell block material by using ISH. More consistent staining and better signal integrity was obtained with the Ventana BenchMark ULTRA than the BenchMark XT.