Liquid biopsy in metastatic breast cancer

The spread of single tumor cells shed by the primary tumor has been observed in most solid carcinomas and is generally associated with poor clinical outcome. Tumor cells detected in the peripheral blood are commonly referred to as circulating tumor cells (CTCs) and are seen as possible precursors of metastatic disease. Beyond CTCs, circulating tumor DNA and non-coding RNA are increasingly the focus of translation cancer research. In metastatic breast cancer (MBC), elevated levels of CTCs have been confirmed as an independent prognostic factor. While detection of elevated counts after the start of systemic therapy predicts poor response, it is unclear which treatment strategy should be offered in the case of CTC persistence. Currently, the main potentials of blood-based diagnostics in BC are therapy monitoring and liquid biopsy-based treatment interventions. Recently, the first positive study on CTC-guided therapy choices in hormone receptor positive HER2 negative MBC was published. In the present review, we discuss the current data and potential clinical application of liquid biopsy in the metastatic setting.