Phenotype of circulating tumor cell: face-off between epithelial and mesenchymal masks

Most patients with cancers died of distant metastasis. It is always difficult to find cancer metastasis in early time, let alone to prevent or cure it. Currently, oncologists place high hopes on circulating tumor cell (CTC), which, compared to current imaging methods, is found more sensitive for early metastasis. Recently, techniques for CTC enrichment and identification are developing quickly. However, there are great challenges in the clinical interpretation of CTC assessments. Increasing studies have shown the heterogeneity of CTCs, which may play different roles in cancer metastasis. Epithelial-mesenchymal transition is not only the main mechanism of the cancer cells invading the circulation system but also a distinguished characteristic of CTCs. Investigators are trying to differentiate specific subgroups of CTCs that are truly responsible for cancer metastasis. Here, we reviewed the current evidences on epithelial-mesenchymal transition of CTCs from perspectives of enrichment methods, biology, and its subgroups.