Correlation Of Trastuzumab Treatment Benefit With Quantitative Her2 Expression Levels In Her2 Positive Metastatic Breast Cancer.

593 Background: The HERmark Breast Cancer Assay (Monogram Biosciences) is used as an adjunct to HER2 immunohistochemistry (IHC) and HER2 fluorescence in-situ hybridization (FISH) to determine the HER2 status of breast cancer (BC). Based on the high degree of concordance of HERmark with centrally determined HER2 status by IHC/FISH, it is expected that HERmark positive BC patients would realize greater benefit from trastuzumab treatment than HERmark negative BC patients. In the current study, this hypothesis was confirmed using a cohort comprised of: (1) HER2 positive BC patients treated before the availability of trastuzumab, (2) trastuzumab-treated HER2 positive BC patients and (3) HER2 negative BC patients. Methods: HERmark was performed retrospectively on formalin-fixed, paraffin-embedded tumors derived from 305 metastatic breast cancer patients in the Munich Cancer Registry with a median follow up of 10 years. Cases were evenly divided between trastuzumab-untreated HER2 positive (n = 91), trastuzumab-t...