Modalities to enumerate circulating tumor cells in the bloodstream for cancer prognosis and to monitor the response to the therapy

Certain malignant cells may detach from the primary tumor and enter the vascular system, forming so-called circulating tumor cells (CTCs). Clusters of malignant cells associated with other cell types can also be observed in the peripheral blood of oncological patients. Such cell clusters are termed circulating tumor microemboli (CTM). The isolation and quantification of CTCs and/or CTM from blood samples allow for an accurate prognosis of the clinical course of the disease and to monitor the response to therapy. Current protocols rely on epithelial markers for the isolation of CTCs and/or CTM from hematopoietic cells. However, epithelial markers may be silenced during the progression of the epithelial-mesenchymal transition, which regulates the detachment and migration of malignant cells from the primary tumor. This review summarizes the achievements and challenges of various modalities for the isolation, enrichment, analysis and enumeration of CTCs and/or CTM, in order to assess the advancement of the disease and the response to therapy.