When cancer spreads beyond its original location, treatment becomes much more difficult.
Doctors often depend on imaging scans like CT scans and MRI scans to see where tumors have moved and how large they are. But scans cannot always detect every cancer cell, especially in the early stages of spread.
A new international study now suggests that a blood test may help doctors detect hidden cancer activity and choose better treatments for patients whose cancer has only spread to a small number of places.
The research was presented at ESTRO 2026, the Congress of the European Society for Radiotherapy and Oncology, and was also published in the Journal of Clinical Oncology.
The study was led by researchers from The University of Texas MD Anderson Cancer Center and The Mayo Clinic. Scientists say the findings may help doctors personalize cancer treatment more accurately in the future.
The research focused on a condition known as oligometastatic cancer. This happens when cancer spreads to only a few locations in the body instead of spreading widely.
For years, doctors have believed that patients with oligometastatic cancer may respond better to focused treatments because the disease is still relatively limited.
These patients are often treated using a combination of drug therapy and highly targeted radiation therapy aimed directly at tumors.
Radiation therapy works by using high-energy beams to destroy cancer cells. Modern radiation technology allows doctors to target tumors with great precision.
However, one major challenge remains. Doctors do not always know which patients are most likely to benefit from adding radiation therapy to their treatment plan.
To solve this problem, researchers studied a blood marker called circulating tumor DNA, or ctDNA.
Cancer cells release tiny pieces of DNA into the bloodstream as they grow and break apart. Scientists can measure this tumor DNA using a blood sample.
Researchers believe ctDNA testing may provide a more sensitive way to monitor cancer compared to scans alone because it may detect cancer activity before tumors become visible on imaging tests.
The clinical trial involved 237 patients with different types of cancer, including pancreatic cancer, breast cancer, kidney cancer, prostate cancer, and several other forms of cancer.
Each patient had between one and five metastatic tumors.
The patients were randomly divided into two groups. One group received standard drug therapy alone, while the second group received both drug therapy and targeted radiation therapy.
Blood samples were taken at several points during the study, including before treatment began, after three months, and later if the cancer progressed.
The researchers found that patients who had tumor DNA in their bloodstream at the start of treatment were more likely to experience continued cancer growth and shorter survival.
The study also showed that patients receiving radiation therapy were more likely to clear ctDNA from their blood.
This was an important discovery because patients whose ctDNA disappeared after treatment had much better outcomes than patients whose blood still contained tumor DNA.
Researchers believe this may mean the blood test can help doctors determine whether treatment is actually working.
The findings also suggest that if ctDNA remains in the blood after therapy, there may still be hidden cancer cells inside the body that scans are unable to detect.
Scientists say this information could help doctors act earlier by changing medications, increasing monitoring, or adjusting radiation treatment.
Dr. Alex D. Sherry explained that ctDNA may eventually help doctors identify which tumor sites are responding to treatment and which areas may be developing resistance.
This could allow cancer treatment to become far more precise and individualized.
Experts not involved in the study also praised the findings.
Professor Matthias Guckenberger from University Hospital Zurich said the blood test could become an important partner to traditional imaging scans.
He explained that ctDNA testing may offer a more detailed understanding of how cancer spreads and responds to treatment over time.
Because the test only requires a blood sample, it is also considered minimally invasive compared to many other medical procedures.
Researchers say the study is especially important because it was one of the largest randomized controlled trials ever performed in patients with oligometastatic cancer.
Randomized trials are considered highly reliable because patients are assigned to treatments randomly, reducing bias in the results.
Still, scientists caution that more research is needed before ctDNA testing becomes a routine part of cancer care.
Doctors will need additional studies to determine exactly how treatment should change based on ctDNA results and whether using the test routinely will improve long-term survival.
Another important question is cost. Advanced DNA testing can still be expensive, and researchers will need to study whether the benefits justify widespread use.
Even so, experts believe the technology has major potential.
The findings represent another step toward precision cancer medicine, where treatments are customized based on each patient’s unique cancer biology rather than relying only on tumor size or scan results.
In the future, a simple blood test may help doctors detect hidden cancer earlier, monitor treatment more accurately, and improve survival for many patients facing cancer spread.
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Source: The University of Texas MD Anderson Cancer Center.
