Figure 5.

CTC isolation and characterization are key challenges for their clinical utility. (A) The complexity of the primary tumor microenvironment affects tumor progression and CTCs and CTC cluster formation. CTCs detach from the primary tumor as single cells and clusters, shed into the bloodstream, and migrate to colonize in distant organs, known as metastasis. The predictive significance of CTC analysis includes the detection of CTCs in the early stage of malignancy in resectable solid tumors.The CTC isolation methods are based on the differences between target CTCs and blood cells. These differences include physical properties (size, density, electrical charge, and deformability) and specific biological characteristics (expression of surface protein markers and viability).The specific features of CTCs make their isolation methods very demanding: (B) The low concentration of CTCs in the blood (estimated as 1–10 single CTCs and 1 CTC cluster per 1 mL of blood) and their extremely short life span in circulation (a few hours for CTCs and even shorter for CTC clusters); (C) CTC heterogeneity and cellular plasticity due to the epithelial–mesenchymal transition (EMT), in which the expression of characteristic epithelial markers, the target of most immunoaffinity-based CTC isolation methods, is lost; (D) in the circulation system, CTCs are subject to blood shear force damage and immune attack. Isolation of the viable, pure, and intact cells is an important aspect of CTC analysis as it enables CTC proliferation in cell culture and downstream analysis.