Scientists say they've developed a blood test that can detect the possible relapse of early stage breast cancer that has been treated, identifying returning tumors a full eight months before they would show up on medical scans.

The test involves identifying cancer DNA the tumors shed into the patient's bloodstream to monitor whether the cancer shows signs of returning, researchers say.

Although still in the experimental stage and likely to be years away from common use, such a test could help personalize breast cancer treatments for patients, they say.

"We have shown how a simple blood test has the potential to accurately predict which patients will relapse from breast cancer, much earlier than we can currently," says Nicholas Turner at the Institute of Cancer Research in London, lead author of a study appearing in the journal Science Translational Medicine.

The test, the researchers explain, could help with one challenge doctors face in how to treat breast cancer — deciding who is most at risk of developing a secondary cancer following initial treatment.

"If we can identify better who is at risk of relapse, we can direct treatments to prevent relapse specifically to them," Turner says. "Women who still have tumor DNA detectable have a high risk of going on to relapse."

In the study, the researchers followed 55 patients who had undergone chemotherapy followed by surgery for early stage breast cancer, taking blood samples from them regularly for two years afterward.

They analyzed the samples for tumor-specific mutations showing up in the blood.

Cancer eventually returned in 15 of the patients and was accurately predicted through the blood tests in 12 of them around eight months before the returning cancer was detectable by conventional imaging techniques, the researchers reported.

In some of those patients, the tests revealed known drug-resistant mutations, giving doctors a chance to design new treatments to specifically target those mutations, they said.

Ninety-five percent of breast cancer cases are diagnosed early, but the ability to determine whether treatment has removed all signs of the disease from the body is vital in preventing the return of tumors, they point out.

By identifying small amounts of returning cancer before it spreads more widely, the test could offer "a window of opportunity to treat the cancer while it is still theoretically curable," says Dr. Tilak Sundaresan, an oncologist with Massachusetts General Hospital Cancer Center in Boston, who co-authored a commentary accompanying the published study.