Whether to determine HER2 status for breast cancer in the primary tumour or in the metastasis

Trastuzumab has substantially changed the prognosis of breast carcinomas. As HER2 over-expression/amplification is a prerequisite for treatment with trastuzumab, an accurate assessment of HER-2 status is the first step for successful treatment. In metastatic breast cancer, we routinely assess HER2 expression in the primary tumour, assuming that HER2 status remains stable through cancer progression. However, it is frequent to find reports that describe discordance between HER2 expression in primary and metastatic tumours. The aim of this paper was to verify whether HER2 status of breast carcinomas is maintained in the corresponding axillary metastasis. Immunohistochemistry was performed on 52 breast carcinomas and their matched axillary metastasis. HercepTest results were concordant in 46 out of 52 cases (88.5%). FISH proved that the differences observed were clinically relevant in only one of the 52 cases studied (98% concordance). We concluded that HER2 status was stable during axillary metastatic progression. Evaluation of gene HER2 status in axillary metastasis rather than in the primary can be useful in certain situations, e. g., small invasive component intimately mixed with in situ component and difficult to recognize in dark field, no tumor after biopsy, or axillary relapse (in this case we can find occasional de novo amplifications susceptible to trastuzumab treatment).