Blood test for tumour DNA could help treatment for spreading cancer

A blood test that detects circulating tumour DNA (ctDNA) in the bloodstream can help doctors choose the most effective treatment options for patients whose cancer has started to spread

The findings, from one of the largest randomised controlled trials of its kind, were presented at the ESTRO 2026 Congress and published in the Journal of Clinical Oncology.

Beyond scanning: A new way to measure spread

The clinical trial focused on patients with oligometastatic cancer, a stage where the disease has just begun to spread to between one and five new sites in the body. Traditionally, doctors diagnose and evaluate this stage simply by counting the number of visible tumours on X-ray, CT, or MRI scans.

However, researchers noted that counting lesions on a scan might not be the most effective way to identify which patients will respond well to treatment. The trial investigated whether measuring ctDNA—microscopic fragments of the tumour shed directly into the bloodstream—could offer a more precise look at the cancer’s activity, predict therapy responses, and provide an accurate long-term prognosis.

Trial design and findings

The study involved 237 patients divided into six sub-groups based on cancer type, including pancreatic, breast, kidney, and prostate cancers. The participants were randomly assigned to receive either standard drug therapy alone or a combination of drug therapy and high-precision radiation therapy directed at the new tumour sites. Blood samples were collected at the start of the trial, after three months, and again if the cancer advanced further.

The researchers discovered that patients who had detectable tumour DNA in their blood at the beginning of the trial faced a higher risk of continued cancer growth and lower survival rates. However, the data also showed that patients who received targeted radiotherapy alongside their drug therapy experienced a significantly better clearance of ctDNA from their blood.

Implications for personalised care

The clearance of ctDNA from the bloodstream strongly correlated with superior patient outcomes and improved survival. Conversely, if tumour DNA remained detectable in the blood after treatment, it served as an early warning sign that the cancer was highly aggressive, developing resistance, or hiding in areas not caught by standard imaging scans.

The research team hopes that ctDNA testing will eventually be used to screen for the best candidates for radiotherapy. Future studies will look into whether doctors should change a patient’s drug regimen if ctDNA remains present after treatment, using the blood test to distinguish exactly which cancer sites are responding and which are becoming resistant.

Matthias Guckenberger, President of ESTRO, noted that this non-invasive blood test could effectively support established imaging techniques. By offering a more refined method to track how a cancer behaves, the test can help doctors determine exactly how and when to deploy radiotherapy, altering treatments when a therapy fails and offering reassurance when it succeeds.