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ctDNA, Recurrence Risk, and Treatment Decisions: Detection Is Not the Final Step
ctDNA may identify molecular signs of remaining cancer. Trials still must show when changing treatment based on the result improves outcomes.
Original commentary from the Cancer Explained editorial team.
Please note: this page is educational only — it is not medical advice, and it does not speculate about anyone’s health beyond reliable public reporting. For questions about your own health, talk with your healthcare team.
The question has moved on
For several years the research question about circulating tumor DNA was simple: does a blood signal after treatment predict recurrence? That is largely settled in some cancers.
The harder question now is what to do about it. Should treatment be added when the test is positive? Can it be safely skipped when the test is negative? Those are decisions, and decisions need their own evidence.
This page explains official sources. It is not medical advice and does not suggest a test or treatment.
Prognostic and predictive are not the same word
NCI's framing of tumor markers separates these jobs clearly. A marker can help estimate prognosis. A different kind of claim is that a marker shows which treatment will help.
A prognostic test sorts people by risk. A predictive test identifies who benefits from one particular action. A test can be excellent at the first and useless at the second. Our page on circulating tumor DNA covers what the test measures, and minimal residual disease covers the state it is trying to detect.
A real study, and exactly what it showed
A study called BESPOKE CRC is a useful case, because NCI has described both its findings and its limits.
The study looked at people who had surgery for colorectal cancer that had begun to spread beyond its original location. Among patients whose ctDNA test was positive, those who received chemotherapy after surgery lived longer without the cancer returning than those who did not.
Among patients whose test was negative, there was no evidence that chemotherapy helped them live longer without a recurrence.
Why that matters is captured by Carmen Allegra of NCI. About 60% of people with this type of colorectal cancer are cured by surgery alone. As he put it, we do not want to expose those patients to chemotherapy and its side effects if we do not have to.
What the study did not do
This is the part worth reading twice.
The ctDNA measurements were not used to choose treatment for individual patients in the first place. Results were given back to participants and their care teams afterward, to see whether plans changed. The study was funded by the company that makes the test it used.
The lead investigators did not overclaim. Their conclusion was that the findings are strong enough to give assurance that ongoing trials of ctDNA-driven treatment decisions are safe and reasonable to pursue. That is a statement about running the next trial, not about changing practice today.
Two ctDNA tests are already marketed for monitoring colorectal cancer recurrence. Being available for purchase is not the same as being proven to guide a treatment decision.
What a decision trial looks like
A trial designed to answer the treatment question states up front which direction it is testing. Does a positive result add treatment? Does a negative result reduce it? Does either change the monitoring schedule?
Then both groups need the same clear follow-up, and the report needs to cover recurrence, survival, side effects, and how much treatment people actually received.
There is a design trap in de-escalation studies. If a trial is not built to rule out an important loss of cancer control, a conclusion that "less treatment was fine" may simply reflect a study too small to detect the harm. The phases of clinical trials explains what each stage of testing is designed to answer.
The risk runs in both directions
- Treating a molecular signal early may add side effects without proven benefit.
- Withholding treatment on a negative test may miss disease the blood sample never picked up.
- A test that predicts recurrence is not automatically a test that should guide therapy.
- A result that conflicts with scans and pathology has to be interpreted, not simply obeyed.
Questions before a blood result changes a plan
- Is this test prognostic, predictive, or both, for my cancer?
- Has a randomized trial shown benefit from acting on the result?
- Who funded the study behind the headline?
- What happens if the blood result and my scans disagree?
How this article was prepared
An AI-assisted editorial system helped prepare this page. No named medical reviewer has reviewed it unless one is listed.
The National Cancer Information Foundation publishes Cancer Explained. This page is for learning. It is not medical advice and does not suggest a test or treatment.
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Put the story in context
Prevention, possible warning signs, screening, and diagnosis
This story relates to ctDNA-guided cancer care. The information below is general: it does not reveal anything else about a public person’s health, and not every point applies to every cancer. Personal advice depends on age, symptoms, family history, exposures, and medical history.
Prevention and risk reduction
Not every cancer can be prevented. Avoiding tobacco, protecting skin from ultraviolet radiation, limiting alcohol, staying active, and receiving recommended HPV or hepatitis B vaccination can lower the risk of certain cancers. A risk factor is not a prediction or a cause in one individual.
Symptoms and possible early signs
Possible signs vary and are often caused by conditions other than cancer. Changes worth discussing include a new lump, unexplained bleeding or weight loss, a persistent cough, lasting bowel or bladder changes, a changing skin spot, or symptoms that persist or worsen. Some early cancers cause no symptoms.
Screening and early detection
Screening looks for certain cancers before symptoms begin. Recommended tests exist only for some cancers and depend on age and risk. Screening can have benefits and harms; it is not the same as evaluating a new symptom, and there is no single routine scan or blood test that reliably screens for every cancer.
How cancer is diagnosed
Diagnosis may involve a history and exam, imaging, laboratory tests, and often a biopsy. Pathology can identify the cancer type and may test biomarkers that guide treatment. Symptoms, screening results, tumor markers, or online stories alone cannot confirm cancer.
A public story may encourage questions, but it should not be used to estimate your risk or choose testing. Contact a healthcare professional about a persistent or concerning change. Seek urgent care for severe or rapidly worsening symptoms.