Unit
- Definition: Circulating tumor DNA (ctDNA) refers to fragments of double-stranded DNA released into the bloodstream by tumor cells through apoptosis, necrosis, or active secretion. These fragments carry tumor-specific genetic alterations including point mutations, copy number changes, and structural variants.
- Origin: ctDNA originates from primary tumors, metastatic sites, and circulating tumor cells. The quantity of ctDNA correlates with tumor burden, though shedding varies by cancer type, location, and vascularity.
- Physical characteristics: ctDNA fragments are typically shorter (90-150 base pairs) than cell-free DNA from normal cells (often longer than 200 base pairs). This size difference is exploited in some analytical methods to enrich for tumor-derived fragments.
- Biological context: ctDNA comprises a fraction of total cell-free DNA circulating in plasma, which also includes DNA from hematopoietic cells, other normal tissues, and fetal DNA in pregnant women. The mutant allele fraction of ctDNA is often very low (less than 0.1% in early-stage disease).
- Potential impact: ctDNA analysis provides a minimally invasive liquid biopsy that can capture the genetic heterogeneity of a patient's cancer across multiple metastatic sites, offering advantages over single-site tissue biopsy.
- Current status: ctDNA testing is clinically validated for non-small cell lung cancer (NSCLC) to identify EGFR mutations, for colorectal cancer monitoring, and is increasingly used in clinical trials as a surrogate endpoint.
- Challenges: The low abundance of ctDNA in early-stage cancers requires ultra-sensitive detection methods. Distinguishing true tumor-derived variants from clonal hematopoiesis of indeterminate potential (CHIP) is a critical interpretive challenge.
- Analytical pipeline: Typical ctDNA analysis involves blood collection in specialized tubes, plasma separation, DNA extraction, library preparation, next-generation sequencing (NGS), bioinformatic analysis, and variant interpretation.
Scope of Practice
Role: Clinical Laboratory Director in a Liquid Biopsy Lab
Within scope:
- Selecting and validating ctDNA extraction methods suitable for low-input DNA
- Establishing protocols for plasma separation and storage to minimize DNA degradation
- Developing and verifying NGS panel designs for ctDNA variant detection
- Implementing quality control measures to monitor contamination and carryover
- Interpreting ctDNA results in the context of clinical history and other laboratory data
Outside scope:
- Making treatment decisions based on ctDNA results without clinical correlation
- Diagnosing cancer solely from ctDNA findings without tissue confirmation
- Reporting variants of unknown significance (VUS) as actionable without functional validation
- Altering established clinical guidelines without regulatory approval
- Communicating results directly to patients without ordering clinician involvement
Quick Revision
- ctDNA is tumor-derived DNA fragments in blood, originating from apoptosis, necrosis, or secretion.
- ctDNA is typically shorter than normal cell-free DNA and represents a small fraction of total cfDNA.
- ctDNA analysis enables non-invasive tumor genotyping and monitoring, but requires sensitive detection methods.
- Interpretation must account for clonal hematopoiesis and other non-tumor sources of variants.
- Clinical laboratory directors are responsible for method validation and quality assurance, not direct patient management.