All cells in the human body constantly undergo cell division and cell death. During this process, tiny fragments of DNA are released from cells into the bloodstream. In cancer patients, tumor cells also release DNA into the bloodstream. This circulating cell-free DNA (cfDNA) originating from tumor cells is known as circulating tumor DNA (ctDNA). Analyzing ctDNA can provide crucial insights into the status and evolution of a patient's cancer.
What is ctDNA?
Cancer cells undergo apoptosis or programmed cell death similar to healthy cells. However, the rate of cell turnover is much higher in tumors compared to normal tissues. During apoptosis, DNA fragments are released from the dying cells. This DNA circulates freely in the bloodstream without being contained inside any cells. The ctDNA shares the same molecular characteristics as the tumor it originated from such as mutations, methylation patterns and fragile sites. Circulating Cell-Free Tumor DNA makes up a very small fraction of the total cfDNA present in blood, usually less than 1%. But since it carries tumor-specific molecular signs, ctDNA provides a "liquid biopsy" of the tumor.
Detecting Circulating Cell-Free Tumor DNA
Advanced genome profiling techniques allow sensitive detection and characterization of ctDNA. Targeted methods screen for mutations already known to be present in the primary tumor or metastases. Whole-genome sequencing and whole-exome sequencing analyze the entire ctDNA for unknown mutations. Digital PCR provides extremely high sensitivity to detect even very low quantities of ctDNA. Combining different techniques enables simultaneous analysis of multiple genomic biomarkers and improves ctDNA detection rates. Non-invasive ctDNA analysis only requires a simple blood draw, making it convenient for longitudinal monitoring and serial sampling.
Clinical Applications
Analyzing ctDNA has numerous applications in oncology. It can be used to diagnose cancer early, guide treatment decisions, detect relapse and monitor response to therapy in real-time. Pre-treatment ctDNA levels have been shown to predict prognosis and risk of recurrence in several cancers. Quantifying ctDNA after initial treatment helps detect minimal residual disease left behind by surgery or chemotherapy. Emergence of new ctDNA mutations during therapy provides early warning of acquired resistance. Serial ctDNA monitoring complements imaging tests to track tumor burden changes with higher sensitivity. ctDNA profiling also helps understand tumor heterogeneity and identify targetable genetic alterations for personalized treatment options.
Challenges in ctDNA Analysis
Despite significant progress, there are still hurdles to overcome before ctDNA testing can be routinely used in clinical practice. Technical challenges include developing very sensitive methods to detect the tiny amounts of ctDNA present in blood against a high background of normal cfDNA fragments. ctDNA detection rates depend on tumor type, stage and size, with limited yield in early or low biomass cancers. Interpreting ctDNA results also requires a clear understanding of how tumor biology correlates with ctDNA kinetics and levels over time. Large, well-designed studies are still needed to validate ctDNA metrics of prognosis, recurrence and treatment response for different cancer types. Standardization of analytical protocols and reporting criteria will facilitate multicenter ctDNA trials and ultimate clinical integration.
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