Fig. 1.

Trends in CTC evaluation. Number of clinical studies using each type of CTC evaluation between 1999 and 2017. Data were obtained from ClinicalTrials.gov by searching the terms “circulating tumor cells” and “cancer”, revealing 493 clinical programs involving CTC evaluation. The use of CTCs in clinical trials has been increasing over the past 20 years. These trials include enumeration, characterization, or a combination. Molecular characterization (blue), which encompasses genetic and proteomic characterization of CTCs, shows a steady increase and signals a trend in cancer therapeutic development