The human epidermal receptor-2 (HER-2) is overexpressed or amplified in 15% to 25% of breast
cancers. Determination of HER-2
tumor status offers clinically useful information, as it selects patients who may benefit from treatment with
trastuzumab, the
monoclonal antibody against HER-2. Currently approved methods for HER-2 testing include immunohistochemistry or fluorescent in situ hybridization using
tumor tissue. A fragment of HER-2 composed of its extracellular domain (ECD) can also be detected in the serum of some patients with
breast cancer. As an easily accessible
tumor marker, it could offer additional useful prognostic or predictive information. This review will briefly address the biology of the circulating HER-2 ECD and discuss the evidence to support the role, if any, for measuring HER-2 ECD levels in women with
breast cancer. In particular, we focus on the value and limitations of serum ECD in both early and advanced
breast cancer in the following clinical contexts: as a marker of HER-2
tumor tissue status; clinical implications of raised levels in women who have a
tumor not overexpressing HER-2; as a prognostic
indicator and as a predictor of response to treatment; and as a monitoring tool for early recurrence. On the basis of our review of the literature, we conclude that there is currently insufficient evidence to support the use of serum HER-2 ECD in the routine management of individual patients with
breast cancer. This conclusion is in agreement with the 2007 American Society of Clinical Oncology guidelines on the use of
biomarkers in
breast cancer.