Circulating tumor cells in metastatic breast cancer: Ready for clinical practice?

Circulating tumor cells (CTCs) can be routinely detected in peripheral blood of patients with solid malignancies. In metastatic breast cancer, presence of 5 or more CTCs per 7.5 ml blood has been shown to predict poor clinical outcome with level I evidence. Further, persistently high CTC counts are associated with inadequate response to therapy. However, it is unclear which therapeutic strategy should be recommended for these patients, since switching to an alternative chemotherapy regimen has not proven beneficial in the SWOG0500 study. In the context of liquid biopsy based interventions, several research groups have explored targeted therapies with DETECT III trial results presented at the SABCS 2020 being the latest "breaking news" in this setting. In the present review, we present and comment on the current evidence regarding CTCbased diagnostics in patients with metastatic breast cancer.