A new immunohistochemistry‐based assay SBrC5 classifies invasive breast cancer subtypes ‐profiling five biomarkers in one single test

Obtaining expression status of ER, PR and Her2 is essential for guiding treatment and predicting outcome for breast cancers. It has been reported that by adding cytokeratin 5/6 and EGFR to the above biomarkers, a five‐biomarker breast cancer panel had been validated for identifying the basal‐like subtype with superior prognostic values than the triple negative subtype1. The current study aims to evaluate a new immunohistochemical assay, named Subtyping Breast Cancers 5‐in‐1 Assay (SBrC5) for classifying breast cancer subtypes. A cocktail of ER, PR, Her2, cytokeratin 5/ 6 and EGFR antibodies (SBrC5 Cocktail) and the MultiVision Detection Kit (two colors, red and blue) were used. The assay was performed on a breast cancer tissue microarray of 75 cases. Both color and cellular localization of the staining was used for the result interpretation. There were three staining clusters: 1) Red Nuclei indicated ER or PR positive; 2) Red Membrane indicated Her2 expression; and 3) Blue Membrane or Cytoplasm indicated EGFR or cytokeratine 5/6 positivity respectively. The concordance for the assay when compared to the individual tests for each biomarker were 94.1%, 93.8% and 92.4% for Her2, combined ER and PR, and combined cytokeratine 5/6 and EGFR respectively. It is concluded that the SBrC5 assay is able to provide sufficient information for classifying breast cancer subtypes with only a single IHC test.