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.

Table 2 Liquid biopsy and early detection of gastric cancer. Summary of the main published studies[146-148]

Role	Structure	Approach	Accuracy	Findings/challenges
Diagnosis	CTCs	Count	S: 42-85%; E: 90%-99%	Low sensitivity; diverse cutoffs
		CD44+	S: 92%; E: 100%	Small sample size
	cfDNA/ctDNA	Methylation	S: 50%-69%; E: 75%-92%	Diverse markers
		Quantification	S: 79%-96%; E: 91%-94%	Diverse methodology
	RNA	MiRNA	S: 67%-99%; E: 66%-95%	Diverse markers
		CircRNA	S: 78%-89%; E: 45%-84%	Diverse markers
		LncRNA	S: 65%-84%; E: 53%-87%	Diverse markers
	EVs RNA	MiRNA	S: 26%-95%; E: 56%-100%	Up or down expression; diverse markers
		CircRNA	S: 41%-87%; E: 68%-98%	Down expression; diverse markers
		LncRNA	S: 70%-88%; E: 60%-98%	Up expression; diverse markers
	Proteins	TTF1-3, CDH17, TFF3, and TXNRD1	S: 62%-96%; E: 57%-83%	Diverse markers; panels

CTC: Circulating tumor cells; S: Sensitivity; E: Specificity; EV: Extracellular vesicle.

Table 3 Liquid biopsy, gastric cancer prognosis and therapy. Summary of the main published studies[146-148]

Role	Structure	Approach	Association	Findings/challenges
Prognosis	CTCs	Count	↓OS/D-RFS	Diverse cutoffs
		EpCAM, CEA, CD44, CD133, TWIST, ploidy, FR, PD-1	↓OS/D-RFS	Diverse methodology
	cfDNA/ctDNA	Quantification	↓OS/D-RFS	Diverse methodology
		Amplification: BRAF, FGFR2, MET; mutation: TP53, ARAF	↓OS	Diverse methodology
		Methylation	↓OS/DFS	Diverse markers
	EVs RNA	MiRNA	↓OS/D-RFS	↑ or ↓ expression; Diverse markers
		CircRNA	↓OS/RFS	↑ or ↓ expression; diverse markers
		LncRNA	↓OS/DFS	↑Expression; diverse markers
Predictive	CTCs	Count	Response to ST
		HER2, PD-L1 status	Trastuzumab resistance/IT
	cfDNA/ctDNA	Quantification	Response to ST-CT/anti PD-1
		Panels Amplification: HER2; mutation: PIK3CA, NF1, HER2, EGFR, TP53, BRCA MSI	Targeted therapy; response to ST; trastuzumab resistance; treatment monitoring; IT

CTCs: Circulating tumor cells; FR: Folate receptor; OS: Overall survival; D-RFS: Disease free survival/relapse free survival; EV: Extracellular vesicle; ST: Systemic therapy; IT: Immunotherapy; CT: Chemotherapy; MSI: Microsatellite instability.