HER2 assessment on core biopsy specimens using monoclonal antibody CB11 accurately determines HER2 status in breast carcinoma

Aims: HER2 status is a prognostic factor in breast carcinoma and predicts response to trastuzumab therapy. Trastuzumab is effective in the neoadjuvant, adjuvant and advanced disease settings. Knowledge of HER2 status early in the diagnostic and therapeutic process is vital for treatment planning. HER2 analysis is usually carried out by immunohistochemistry or fluorescence in situ hybridization (FISH). The aim of this study was to establish whether HER2 immunohistochemistry using monoclonal antibody CB11 and carried out on the initial diagnostic core biopsy specimen accurately predicts HER2 amplification status. Methods and results: This is the largest study to date in which HER2 protein expression has been assessed by CB11 immunohistochemistry on the diagnostic core biopsy specimen and correlated with the result of FISH. Using FISH as the definitive HER2 status, we studied 568 invasive breast cancers using CB11 immunohistochemistry on core biopsy. This analysis had a sensitivity of 99.4%, specificity of 93.9%, false-positive frequency of 3.9% and false-negative frequency of 1.1%. These data are as good as those obtained from analysing resection specimens alone in UK national reference centres. Conclusions: CB11 immunohistochemistry accurately predicts HER2 amplification status and can be reliably carried out on core biopsy specimens of breast carcinoma.