Figure 2.

Scheme of metastatic progression and potential use of liquid biopsy. In the bloodstream, there are different circulating tumour cell (CTC) subpopulations. Those that have survived the therapy and become resistant are responsible for the recurrence and progression of the disease to distal locations. Liquid biopsy permits the sampling of these and other tumour entities, such as circulating tumour DNA (ctDNA) or extracellular vesicles. As such, serial liquid biopsy throughout therapy is useful for studying the appearance of treatment resistance.