How does a liquid biopsy, which analyzes circulating tumor DNA (ctDNA) or other biomarkers in blood or other bodily fluid, work in patients with advanced cancer, such as lung, breast, or colon cancer?

How Liquid Biopsy Works

Liquid biopsy is a minimally invasive blood test that detects and analyzes tumor-derived materials—primarily circulating tumor DNA (ctDNA), circulating tumor cells (CTCs), extracellular vesicles (EVs), and other biomarkers—shed from primary tumors and metastatic sites into peripheral blood or other body fluids. 1, 2

Core Mechanism and Biological Basis

Liquid biopsy works by capturing tumor-derived components that are released into the bloodstream through several mechanisms 3:

• Circulating tumor DNA (ctDNA) represents fragmented DNA released from dying tumor cells, comprising a fraction of total cell-free DNA in blood 4
• Circulating tumor cells (CTCs) are intact, living cancer cells that have detached from the primary tumor or metastatic sites and entered the bloodstream 1
• Extracellular vesicles (EVs), including exosomes and microvesicles, are lipid-rich particles released through cell activation or degradation, carrying tumor-specific proteins and nucleic acids 5
• Other biomarkers include circulating tumor RNA (ctRNA), microRNAs (miRNAs), long non-coding RNAs (lnRNAs), tumor-educated platelets, and autoantibodies 2

Technical Detection Methods

The actual detection process employs multiple technological platforms 6, 2:

For ctDNA Analysis

• Tumor-informed approaches use customized panels based on known tumor mutations, providing highest sensitivity and accuracy but at greater cost 6
• Tumor-agnostic approaches employ fixed gene panels offering broader applicability at lower cost but with reduced sensitivity 6
• Specific techniques include digital PCR, real-time PCR, mass spectrometry-based platforms, and next-generation sequencing (NGS) panels 6

For CTC Detection

• Surface antigen-based capture utilizes specific markers on CTC surfaces for isolation and characterization 5
• Folate receptor-positive CTC detection in lung cancer demonstrates 72.46-77.7% sensitivity and 82.39-89.5% specificity 7

For Biomarker Analysis

• Immunohistochemistry (IHC), in situ hybridization (ISH), or targeted PCR serve as first-line approaches when tissue is limited 8
• NGS-based comprehensive genomic profiling performed in CLIA-approved laboratories can identify multiple targetable alterations simultaneously 8

Clinical Workflow

The practical implementation follows this sequence 8, 7:

1. Blood sample collection requires approximately 10 mL of blood or other body fluid 9
2. Sample processing isolates the specific biomarker type (ctDNA, CTCs, EVs) using validated extraction methods 2
3. Analysis employs the appropriate detection platform based on the clinical question and biomarker type 6
4. Interpretation requires specialized expertise, as liquid biopsy results are more complex than conventional tissue-based testing 6

FDA-Approved Clinical Applications

Current regulatory approvals are limited but expanding 7, 6:

• EGFR mutation detection in non-small cell lung cancer is FDA-approved for predicting response to EGFR tyrosine kinase inhibitors 6, 7
• PIK3CA mutation testing in breast cancer is validated for selecting patients eligible for alpelisib therapy 6
• HER2 amplification detection and resistance mechanism identification in breast cancer when tissue sampling is not feasible 6

Critical Limitations That Affect How It Works

Understanding these limitations is essential for proper clinical application 8, 6, 7:

• Sensitivity varies dramatically based on cancer stage, tumor burden, proximity of sampling site to disease, and biomarker type 6, 7
• Approximately 25% of NSCLC patients are "non-shedders" with DNA levels below detection limits 6
• Negative results do not exclude tumor mutations and should never be considered definitive 8, 6
• Standardization remains a critical challenge, particularly for NGS-based analyses across different platforms 7, 6

Optimal Clinical Scenarios for Use

Liquid biopsy is most appropriate when adequate tissue cannot be safely obtained or is insufficient for analysis, or when monitoring disease progression in patients unable to undergo repeat biopsies. 8, 6

Specific indications include 8, 7:

• Patients with advanced or metastatic cancer who cannot undergo traditional biopsy 8
• Real-time monitoring during treatment for therapy response or early recurrence detection 6
• Detection of resistance mechanisms during targeted therapy 6
• Situations where tissue biopsy is unsuccessful, insufficient, or patient cannot tolerate invasive procedures 7

Key Distinction from Tissue Biopsy

Liquid biopsy should not replace tissue biopsy when adequate tissue can be safely obtained, initial diagnosis and staging are required, or histologic subtype classification is needed for treatment planning. 8, 7

The fundamental difference lies in the material analyzed 5:

• CTCs represent living, recoverable material that can potentially be cultured and analyzed for downstream applications 5
• EVs and ctDNA represent "dead material" with transient effects on recipient cells, providing molecular information but not viable tumor cells 5
• Tissue biopsy remains the gold standard for lung cancer diagnosis and histologic characterization 7