[HER2 ASCO guidelines. The answer to everything?]

The HER2 gene is amplified and overexpressed in about 15%-20% of breast cancers. For every newly diagnosed breast cancer HER2 testing is a standard routine procedure. This article focuses on a number of issues raised in the context of current HER2 testing in breast cancer. It particularly points out issues arising in the recently published ASCO-CAP (American Society of Clinical Oncology/College of American Pathologists) guideline recommendations for clinical testing of HER2. Despite the significant correlation between HER2 status determination by immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH), standard considerations of laboratory testing, such as test accuracy, reproducibility and precision as well as current data, favor FISH methods over IHC assay methods for the determination of HER2 status. Biological and technical considerations of HER2 testing are also important in clinical practice. For example, HER2 gene amplification is directly linked to the protein expression level in breast cancer; however, the HER2 protein is not consistently analyzed on formalin fixed tissues due to variability in fixation methods/times and the impact of this fixation on HER2 protein antigenicity. FISH is significantly less dependent on tissue fixation artifacts. Hence, FISH is more reproducible between both central and peripheral laboratories than IHC and is more accurate for HER2 measurement, as well as being more strongly correlated with responsiveness to trastuzumab and lapatinib treatment. Until other methods are able to ensure similar test accuracy, reproducibility, precision and predictive value, FISH is recommended as the primary HER2 testing modality for women with breast cancer who are candidates for HER2-targeted therapies.