Cancer becomes much harder to treat once it starts spreading to other parts of the body.
Doctors often rely on scans such as CT scans, MRI scans, and X-rays to see how far the cancer has spread and decide which treatments patients should receive.
However, these imaging methods do not always tell the full story about how active or aggressive the cancer really is.
Now, researchers say a simple blood test may help doctors make much better treatment decisions for patients whose cancer has just started spreading.
The new study was presented at the Congress of the European Society for Radiotherapy and Oncology, also called ESTRO 2026, and was also published in the Journal of Clinical Oncology. Researchers say this is one of the largest randomized controlled studies ever carried out on this topic.
The research was led by Dr. Chad Tang from The University of Texas MD Anderson Cancer Center and presented by Dr. Alex D. Sherry from The Mayo Clinic.
The study focused on patients with something called oligometastatic cancer. This is a stage where cancer has spread outside the original tumor site, but only to a small number of places in the body. Usually, doctors define oligometastatic cancer by counting how many tumors appear on scans.
Patients in this stage may still benefit from aggressive treatment because the cancer has not yet spread widely throughout the body.
Doctors often treat these patients using a combination of drug therapy, such as chemotherapy or hormone treatment, along with targeted radiation therapy directed at the cancer spots.
Radiation therapy uses carefully focused energy beams to damage and destroy cancer cells. Newer forms of radiation can target tumors very precisely while reducing harm to nearby healthy tissue.
Earlier studies already suggested that combining radiation therapy with drug treatment may improve survival for some patients with oligometastatic cancer. However, doctors still face a major challenge: figuring out which patients are most likely to benefit from these treatments.
That is where the new blood test may help.
The test measures something called circulating tumor DNA, also known as ctDNA. Tumors release tiny fragments of their DNA into the bloodstream. By testing a blood sample, doctors can look for these pieces of tumor DNA and measure how much cancer-related material is circulating in the body.
Researchers believe ctDNA may provide a clearer picture of cancer activity than scans alone.
The study included 237 patients divided into six different groups. These included patients with pancreatic cancer, breast cancer, kidney cancer, prostate cancer, and several other cancers. Each patient had between one and five areas where the cancer had spread.
The patients were randomly assigned to receive either drug treatment alone or drug treatment combined with radiation therapy.
Researchers collected blood samples at the beginning of the study, again after three months, and later if the cancer spread further.
The results were important. Patients who had detectable tumor DNA in their blood at the start of the trial were more likely to have worsening cancer and were more likely to die compared to patients whose blood did not contain ctDNA.
The researchers also discovered that patients who received radiation therapy along with drug treatment were more likely to clear ctDNA from their bloodstream.
This finding was especially important because patients whose ctDNA disappeared after treatment had much better survival and cancer control.
Scientists believe this suggests that ctDNA testing may become a powerful new tool for monitoring how well treatment is working.
The study also found that if tumor DNA remained in the bloodstream after treatment, it might mean the cancer is more aggressive, resistant to treatment, or spreading in ways that scans cannot yet detect.
This could help doctors identify problems earlier and adjust treatment plans more quickly.
Researchers hope future studies will explore whether doctors should switch medications or treatment strategies when ctDNA levels stay high after therapy.
Experts who reviewed the study say the findings could help improve cancer care in several ways.
Professor Matthias Guckenberger from University Hospital Zurich, who was not involved in the study, said the blood test could work alongside scans to give doctors a much more detailed understanding of how cancer is behaving inside the body.
Unlike scans, which only show visible tumors, ctDNA testing may reveal hidden cancer activity before it becomes visible on imaging tests.
This could allow doctors to personalize treatment more effectively and avoid unnecessary therapies for patients who may not benefit.
The study also highlights the growing importance of precision medicine in cancer care. Instead of treating every patient the same way, doctors are increasingly trying to tailor treatment based on the biology of each person’s cancer.
Although more research is still needed before ctDNA testing becomes standard practice everywhere, experts say the findings are very promising.
The study is especially important because it involved many patients across several cancer types and used a randomized controlled design, which is considered one of the strongest forms of medical research.
One limitation is that the study mainly focused on patients with a limited number of metastases, so the findings may not apply to patients with widespread cancer.
Still, researchers believe this blood test could become an important new tool for helping doctors choose the best treatment, monitor patient progress, and improve survival in the future.
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