Circulating tumor DNA (ctDNA) is fragmented DNA shed by cancer cells into the bloodstream. These tiny pieces of genetic material carry the same mutations found in the tumor itself, which means a simple blood draw can reveal information about a cancer’s genetic makeup without surgery or a needle biopsy. This concept, often called a “liquid biopsy,” has become one of the most practical advances in cancer care over the past decade.
How Tumor DNA Ends Up in Your Blood
Every cell in your body contains DNA, and when cells die, fragments of that DNA spill into the bloodstream. This happens through several processes. The most recognized is apoptosis, a form of programmed cell death that produces DNA fragments roughly 140 to 180 base pairs long, matching the size of DNA wrapped around the protein spools that organize it inside a cell. Necrosis, a messier form of cell death common in fast-growing tumors that outstrip their blood supply, also releases DNA fragments. Research published in Cell Reports found that necrosis is actually the predominant source of DNA release in tumors exposed to radiation, with apoptosis playing a comparatively minor role in some cancer types.
Cancer cells die constantly, whether from immune attack, poor blood supply, or the effects of treatment. When they do, their DNA fragments enter circulation alongside DNA from healthy cells. The total pool of free-floating DNA in your blood is called cell-free DNA (cfDNA). The fraction that comes specifically from tumor cells is ctDNA.
ctDNA vs. Cell-Free DNA
The distinction matters because ctDNA is a small subset of the total cell-free DNA in your blood. In some patients it makes up less than 0.1% of all circulating DNA; in others with advanced disease, it can exceed 90%. On average, ctDNA exists in much lower quantities than the non-tumor DNA floating alongside it, which makes finding it a bit like picking out a handful of specific needles from a very large haystack.
ctDNA fragments also tend to be physically shorter than regular cell-free DNA. Most cell-free DNA fragments are about 150 to 200 base pairs long, while ctDNA fragments typically range from 50 to 150 base pairs. This size difference is one feature that helps detection technologies distinguish tumor-derived fragments from the background noise of normal DNA.
ctDNA is also less stable. It has a half-life of roughly 16 minutes to 2.5 hours in the bloodstream, meaning the body clears it quickly. That short lifespan is actually an advantage: any ctDNA detected in a blood sample reflects what the tumor is doing right now, not what it was doing weeks ago. It provides something close to a real-time snapshot of tumor genetics.
How ctDNA Is Detected
Finding trace amounts of tumor DNA among a sea of normal DNA requires extremely sensitive technology. Two main approaches dominate. Digital PCR is designed to detect specific, known mutations at very low concentrations. It works well when doctors already know which genetic change they’re looking for. Next-generation sequencing (NGS) casts a wider net, reading many genes at once to identify a range of mutations, but traditional NGS methods struggle to reliably detect variants present below about 1% of the total DNA in a sample. Newer modifications have pushed that threshold down to 0.1% to 0.5%, and specialized research assays can now detect ctDNA at levels as low as 0.003%, or roughly three parts per hundred thousand.
The FDA has approved several ctDNA-based tests for clinical use. FoundationOne Liquid CDx, approved in 2020, is a next-generation sequencing test that screens cell-free DNA from a blood sample for mutations across multiple cancer types. It’s approved as a companion diagnostic, meaning it identifies specific genetic alterations that determine whether a patient is eligible for certain targeted therapies. These include mutations linked to treatment options in ovarian cancer, non-small cell lung cancer, breast cancer, and metastatic prostate cancer. If the blood test doesn’t detect the relevant mutation, doctors are advised to follow up with a traditional tissue biopsy, since ctDNA levels can sometimes be too low to pick up.
Tracking Treatment Response
One of ctDNA’s most valuable uses is monitoring how well cancer treatment is working. Because ctDNA clears from the blood so quickly, rising or falling levels can signal changes in tumor activity days or weeks before imaging scans show any visible difference. If treatment is effectively killing cancer cells, ctDNA levels typically drop. If levels plateau or rise, it can indicate the tumor is not responding or is developing resistance.
A multicenter trial studying patients with locally advanced rectal cancer illustrates how powerful this monitoring can be. Researchers tracked ctDNA levels after each cycle of chemotherapy and classified patients based on how their levels changed over time. Those whose ctDNA cleared quickly or stayed undetectable had a poor response rate of just 12.4%. Patients whose ctDNA was slow to clear or reappeared after initially dropping had a poor response rate of 59.4%. Not a single patient in the high-risk group achieved a major pathologic response. Persistent or rising ctDNA was an independent predictor of poor outcomes, with roughly 12 times the odds of a poor response compared to the low-risk group.
This kind of dynamic monitoring could eventually allow doctors to intensify or change treatment early for patients who aren’t responding, rather than waiting months for scan results.
Detecting Cancer After Surgery
After a tumor is surgically removed, the critical question is whether any cancer cells remain. Traditional imaging often can’t detect microscopic residual disease. ctDNA testing fills this gap through what’s called minimal residual disease (MRD) detection. If ctDNA is still detectable in the blood after surgery, it suggests cancer cells remain somewhere in the body, even if they’re invisible on scans.
In patients with localized lung cancer who underwent surgery with curative intent, specialized ctDNA assays achieved 94% sensitivity and 100% specificity for detecting residual disease. That level of accuracy at such low DNA concentrations (down to roughly 0.003%) makes it possible to identify patients at high risk of recurrence and offer them additional treatment, while potentially sparing low-risk patients from unnecessary chemotherapy.
Practical Limitations
ctDNA testing is not a universal cancer screening tool, at least not yet. In early-stage cancers, tumors are small and shed very little DNA into the bloodstream, so ctDNA can be undetectable even when cancer is present. The proportion of cell-free DNA that comes from the tumor varies enormously between patients and cancer types, and extremely low levels can fall below the detection threshold of even advanced assays.
Blood handling also matters. ctDNA degrades quickly at room temperature, so samples need to be processed and stored properly to avoid losing signal. And while a positive ctDNA result is highly specific for the presence of tumor-derived mutations, a negative result doesn’t guarantee the absence of cancer. That’s why tissue biopsy remains the backup when blood-based testing comes up empty.
Despite these constraints, ctDNA has already changed the landscape for patients with advanced cancers who need genetic profiling to guide targeted therapy, and its role in post-surgical monitoring and treatment adjustment is expanding rapidly.