Table 2. . The principal key characteristic specific or shared among conventional tissue biopsy, circulating tumor cells, circulating-free DNA and exosomes.

Key characteristics	Tissue biopsy	CTCs	cfDNA	Exosomes
Invasiveness	Yes	No	No	No
Availability of the sample throughout the disease process	Re-biopsy needed	Yes	Yes	Yes
Equipment	Special instrumentation	Special instrumentation for cell identification	Simple blood collection	Simple
Isolation and processing	Complex and time consuming	Complex and time consuming	No isolation required, time saving	Easy-to-perform isolation, time consuming
Clinical utility	Early detection; targeted therapy; resistance	Early detection; targeted therapy	Early detection; targeted therapy resistance	Early detection targeted therapy drug delivery

cfDNA: Circulating-free DNA; CTC: Circulating tumor cell.