Circulating Extracellular Vesicles in Cancer Diagnosis and Monitoring

Mammalian cells, including cancer cells, secrete extracellular vesicles. These vesicles are nanosized, bilayered proteolipids with diameters of 50-1,000 nm. It has been suggested that cancer cell-derived extracellular vesicles play diverse roles in cancer progression, which involve invasion, immune modulation, neovascularization, and metastasis. Moreover, their serum levels are significantly elevated in cancer patients compared with normal controls. Recent high-throughput proteomic and transcriptomic studies of these complex extracellular organelles have accelerated the discovery of cancer-specific biomarkers and the development of novel diagnostic tools based on extracellular vesicles. Although many vesicle-associated biomarker candidates have been reported for various types of cancer, few have been validated for clinical use due to preanalytical, technical, temporal, and financial problems. Here, we discuss the potential of extracellular vesicles as sources of biomarkers for cancer diagnosis and monitoring, as well as the limitations and obstacles to adoption of extracellular vesicle-based diagnosis.