Towards a personalized breast cancer treatment approach guided by circulating tumor cell (CTC) characteristics.

Circulating tumor cells (CTCs) can be found in the peripheral blood of patients with different solid tumors, including breast cancer. A CTC count is a strong established prognostic factor in various stages in several tumor types. Besides that, characterization of CTCs is expected to become an invaluable tool to predict treatment response and personalize cancer treatments. Likely, CTCs are shed by different tumor lesions and may therefore provide a comprehensive view of tumor characteristics at a certain time-point, including inter- and intratumoral heterogeneity. Obtained through a simple venipuncture, CTCs could this way serve as a "liquid biopsy". However, isolation and subsequent characterization of CTCs is technically extremely challenging, mainly due to the small number of cells amidst a large majority of leukocytes. A wide range of assays has been developed, but only the CellSearch System(®) (Veridex, Raritan, NJ, USA) has obtained FDA approval for CTC enumeration so far. For characterization purposes, no assay has been validated at all. Nevertheless, the first studies investigating the clinical value of CTC characteristics have been performed. Here, we review these clinical studies. The various techniques used to interrogate CTCs are briefly described and an overview of the clinical relevance of CTC characterization in breast cancer is given.