Liquid biopsy has been introduced as a new diagnostic concept predicated on the analysis of circulating tumour cells (CTCs) or circulating tumour-derived factors, in particular, cell-free tumour DNA (ctDNA). Highly sensitive liquid biopsy assays have been developed that can now be applied to detect and characterize minimal residual disease (MRD), which reflects the presence of tumour cells disseminated from the primary lesion to distant organs in patients who lack any clinical or radiological signs of metastasis or residual tumour cells left behind after local therapy that eventually lead to local recurrence. This application is the new frontier of liquid biopsy analyses, which are challenged by the very low concentrations of CTCs and ctDNA in blood samples. In this Review, we discuss the key technologies that can be used to detect and characterize CTCs in surveillance of MRD and provide a brief overview of similar roles of ctDNA analyses. We then focus on the current clinical data on the use of CTCs and ctDNA in the detection and monitoring of MRD and in obtaining information on therapeutic targets and resistance mechanisms relevant to the management of individual patients with cancer.
