The new advance of exosome-based liquid biopsy for cancer diagnosis

Cancer is a leading cause of death worldwide [ 1 ]. Tissue biopsy has been considered as the standard method for cancer diagnosis [ 2 ]. However, this invasive method may cause potential risks such as cancer metastasis and infection. In addition, due to the heterogeneity of tumors, tissue biopsy can only reflect the pathological status of a certain location, but is hard to reflect the overall status of tumors. Liquid biopsy refers to a technique which diagnoses diseases by detecting circulating tumor cells (CTCs), circulating tumor DNA (ctDNA) and exosomes from cerebrospinal fluid, saliva, pleural fluid, blood, ascites, urine and other body fluids [ 3 ]. Compared with tissue biopsy, liquid biopsy mainly has the advantages of minimal invasion, early diagnosis and dynamic monitoring. At present, the detection of CTCs and ctDNA is of potential significance in the dynamic monitoring of tumor progression, metastasis and drug resistance [ 4 , 5 , 6 , 7 , 8 ]. However, their clinical application still faces many challenges. First, although ctDNA can reflect the mutation of tumor cells, current detection method will be unreliable when variant allele frequency (VAF) is below 0.5% [ 9 ]. Second, the abundance of ctDNA is unstable since its half-lifeis generally less than two hours [ 10 ]. Third, due to the low abundance and heterogeneity of CTCs in blood, the enrichment and detection is challenging, which restrains its clinical application [ 11 ].