Quantitative detection of HER2 protein concentration in breast cancer tissue does not increase the number of patients eligible for adjuvant HER2-targeted therapy.

Human epidermal growth factor receptor-2 (HER2) is overexpressed in 15-20% of breast cancer patients and is associated with an aggressive tumor and a poor prognosis. Currently, patients are selected for adjuvant HER2-targeted therapy based on HER2 status by immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH). In this study, we assessed the clinical significance of tissue HER2 status determined by a quantitative immunoassay using ADVIA Centaur. We investigated the hypothesis that the clinical outcome is worse in a group of patients defined as tissue HER2-positive only by Centaur, but not treated with adjuvant HER2-targeted therapy, compared to patients defined as HER2-positive by IHC/FISH and therefore treated with adjuvant HER2-targeted therapy. Tumor tissue was obtained at primary surgery from 415 breast cancer patients between 2004 and 2010. HER2 status was determined by quantitative immunoassay of fresh-frozen tissue and by IHC/FISH of corresponding paraffin-embedded tissue. We compared the clinical outcome in four groups of patients defined by tissue HER2 status and adjuvant HER2-targeted therapy. The final analysis included 379 patients after a median follow-up of 3.9 years for invasive disease-free survival (IDFS) and 4.2 years for overall survival (OS). The quantitative Centaur assay defined a greater number of patients (100 patients, 26.4%) as HER2-positive than IHC/FISH (63 patients, 16.6%) (P<0.0001). No significant difference in IDFS (P=0.159) and OS (P=0.150) was observed among the four groups of patients. However, in the IHC/FISH-positive group without adjuvant HER2-targeted therapy (group 2), a significantly greater number of events was found compared to the Centaur-positive group without adjuvant HER2-targeted therapy (group 3) for both IDFS (P=0.025) and OS (P=0.020). Quantitative HER2 determination by Centaur did not define a new group of patients eligible for HER2-targeted therapy. Currently, tissue HER2 status defined by IHC/FISH analysis remains the gold standard.