Circulating tumor cells in breast cancer—current status and perspectives

•Presence of circulating tumor cells (CTCs) is associated with worse survival in early and metastatic breast cancer.•The phenotype of CTCs may differ from the primary tumor.•Evaluation of CTCs may serve as a “liquid biopsy” in metastatic breast cancer.•Treatment decisions based on CTC presence and characteristics are currently tested in clinical trials.

The phenomenon of tumor cell dissemination through the blood stream has been known since the 19th century. Circulating tumor cells (CTCs) may be detected in peripheral blood of patients with breast cancer and may serve as a surrogate marker for minimal residual disease. Prognostic relevance of CTCs has already been demonstrated in early and metastatic breast cancer and commercially available detection systems are currently employed in various clinical trials. Since peripheral blood is an easily accessible compartment, serial reevaluation of CTCs is possible and may contribute to better therapy monitoring. Another potential of CTCs lies in the characterization of tumor cells. Expression profiles may differ between CTCs and primary tumor, which may result in different responses to treatment. Assessment of molecular features of CTCs may be an important step for the optimization of adjuvant and metastatic systemic therapy.