Liquid biopsy for gastric cancer: Techniques, applications, and future directions

Table 1 Main non-blood specimens for liquid biopsy studies

Sample	Tumor	Main features
Cerebrospinal fluid	Primary and metastatic CNS	Helpful due to the frequent metastatic seeding, the difficulty in obtaining tissue biopsies from this location, and the obstruction of the blood-brain barrier
Urine and seminal fluid	Prostate, urothelial, renal	Renal cancer-derived structures may be limited by glomerular filtration; cfDNA concentration and fragment size in seminal fluid seem to differ between healthy and prostate cancer patients
Stool	Colorectal	ctDNA extraction can be complex; ctDNA may be present in low concentrations (mixed with DNA from the bacterial flora)
Pleural or peritoneal fluid	Lung, mesothelioma, peritoneal spread	Determination of predictive biomarkers in lung cancer; detection of minimal residual disease or early involvement of the peritoneum
Saliva	Oral cancers	Monitoring of minimal residual disease and treatment response; low ctDNA concentration, limited fragment size
Bile	Biliary tract	Useful in difficult-to-biopsy cases; invasive procedure
Gastric juice	Gastric	Improvement of diagnostic sensitivity; invasive procedure (endoscopy)

CNS: Central nervous system; ctDNA: Circulating tumor DNA; cfDNA: Cell-free DNA.