How does a liquid biopsy work for tumors?

How Liquid Biopsy Works for Tumors

Liquid biopsy analyzes tumor-derived material from body fluids (primarily blood or urine) through minimally invasive sampling, detecting circulating tumor cells (CTCs), circulating tumor DNA (ctDNA), cell-free RNA, and extracellular vesicles that tumors shed into circulation. 1, 2

Core Mechanism and Components

Liquid biopsy works by capturing and analyzing several distinct tumor-derived materials that circulate in body fluids:

Primary Analytes Detected

• Circulating Tumor Cells (CTCs): Living tumor cells that detach from primary or metastatic sites and enter the bloodstream, which can be isolated using surface antigen-based capture methods targeting specific markers on CTC surfaces 1, 2

• Circulating Tumor DNA (ctDNA): Cell-free DNA fragments released from dying tumor cells that carry tumor-specific genetic alterations and mutations 1, 3

• Cell-Free RNA (cfRNA) and microRNAs: RNA molecules including miRNAs and mRNAs that reflect tumor gene expression patterns and can indicate specific cancer types 1

• Extracellular Vesicles (EVs): Lipid-rich particles (including exosomes and microvesicles) released through cell activation or degradation, carrying tumor-specific proteins and nucleic acids 1, 2

Technical Detection Process

The workflow involves several key steps:

• Sample Collection: Typically requires only approximately 10 mL of blood or urine, making it far less invasive than tissue biopsy 4

• Isolation Methods: Surface antigen-based capture for CTCs (achieving 72.46-77.7% sensitivity and 82.39-89.5% specificity in lung cancer), while ctDNA and EVs require specialized extraction techniques 2

• Analysis Platforms: NGS-based comprehensive genomic profiling performed in CLIA-approved laboratories provides the most comprehensive analysis, though targeted PCR and other methods can detect specific biomarkers 5

Clinical Applications and Indications

When to Use Liquid Biopsy

The American College of Physicians and NCCN guidelines recommend liquid biopsy specifically for: 2, 5

• Patients with advanced or metastatic cancer who cannot undergo traditional biopsy due to tumor location, patient comorbidities, or safety concerns

• Real-time monitoring during treatment to detect resistance mechanisms during targeted therapy

• Disease progression monitoring in patients unable to undergo repeat tissue biopsies

• When adequate tissue cannot be safely obtained or is insufficient for molecular analysis

FDA-Approved Applications

• EGFR mutation detection in non-small cell lung cancer for predicting response to EGFR tyrosine kinase inhibitors 2

• PIK3CA mutation testing in breast cancer for treatment selection 2

• Comprehensive genomic profiling for detecting HER2/ERBB2 amplification, MSI status, MMR deficiency, TMB-H, and NTRK gene fusions in gastric cancer 5

Critical Advantages Over Tissue Biopsy

Liquid biopsy offers several mechanistic advantages:

• Overcomes spatial heterogeneity: Unlike tissue biopsy which samples only one tumor location, liquid biopsy captures material from primary tumors and all metastatic sites simultaneously 1, 3

• Serial monitoring capability: Can be repeated frequently to track tumor evolution, treatment response, and emergence of resistance mutations in real-time 3, 4

• Whole-body tumor interrogation: Provides a comprehensive molecular snapshot of all disease sites rather than a single anatomic location 4

Major Limitations and Pitfalls

Sensitivity Constraints

Sensitivity varies dramatically based on cancer stage, tumor burden, proximity of sampling site to disease, and biomarker type, with approximately 25% of NSCLC patients being "non-shedders" with DNA levels below detection limits. 2

• Liquid biopsy is less sensitive than imaging in early tumor stages but more specific for detecting molecular changes 4

• Negative results do not exclude tumor mutations and should never be considered definitive—tissue biopsy remains necessary when liquid biopsy is negative but clinical suspicion is high 2, 5

When NOT to Use Liquid Biopsy

Liquid biopsy should not replace tissue biopsy when: 5

• Adequate tissue can be safely obtained
• Initial diagnosis and staging are required
• Histologic subtype classification is needed for treatment planning

Clinical Workflow Integration

For optimal use in practice:

• Use validated NGS-based assays in CLIA-approved laboratories only—non-validated assays should not guide clinical decisions 5

• Interpret negative results cautiously: A negative liquid biopsy in the setting of clinical or radiographic progression requires tissue confirmation 2, 5

• Best suited for advanced disease settings: Currently most appropriate for metastatic cancer monitoring rather than early-stage screening 5

• Can detect targetable alterations in 69% of advanced gastroesophageal adenocarcinoma patients when tissue is limited, demonstrating clinical utility 5