A blood test might soon be able to tell if women who have been treated for early-stage breast cancer are at risk for relapse.

Women who test positive for tumor DNA after having undergone surgery and chemotherapy are 12 times as likely to relapse compared with women who test negative, according to a study published in Science Translation Medicine today. Given that tumor DNA can be detected months before an actual clinical relapse occurs, it's possible that doctors might one day be able to use this test to predict and then defer a relapse — or even prevent one from happening altogether.

The test detects tumor DNA in the blood

Breast cancer is the most common cancer diagnosis worldwide; it's also the second most common cause of cancer death in women. So far, one of the key challenges in treating women with breast cancer has been working out who is at risk of developing future secondary cancer after they have been treated, says Nicholas Turner, an oncologist at the Institute of Cancer Research in the UK and a co-author of the study. That's why a blood test that detects mutations only found in tumor DNA could be so useful; it's the kind of innovation that could help focus treatments on those who need them the most.

In the study, Turner and his team used the test on 55 women with early-stage, localized breast cancer before they underwent surgery, as well as after they received chemotherapy. Then, the scientists repeated the test on each patient once every six months for a period of two years — or until they relapsed.

The scientists found that women who had tumor DNA in their blood after being treated for early breast cancer had a high risk of future relapse. Of the 15 women who relapsed, 12 had circulating tumor DNA in their blood during the study period.

Women with tumor DNA in their blood had a high risk of future relapse

The test's predictive value is "most exciting in that it opens the door for innovative clinical studies to test [therapies] that might offer a chance of cure" before a clinical relapse becomes evident, write Tilak Sundaresan and Daniel Haber — both cancer researchers at Harvard University who didn't work on the study — in an article accompanying the Science Translational Medicine study. Recurrent forms of breast cancer are "rarely curable," however, so it's unclear whether or not detecting the early signs of a relapse would actually offer a chance for a cure, they write. That's something that scientists will need to work out in future studies.

The study has some limitations. For one thing, the number of women enrolled in the study was small; researchers will have to repeat the study using a much larger group of people to make sure that the blood test really is as predictive as it seems. In addition, the study only sampled blood for a period of two years after treatment. So, scientists don't know if the test could be used to predict a relapse after that period.

"An exciting future."

A blood test that can predict breast cancer relapse would be an easy, non-invasive way of identifying patients that could benefit from additional treatments, but knowing that a patient is at risk of a relapse won't help if doctors can't do anything about it. That's the next step in this work, Turner says.

The researchers have previewed "an exciting future," Sundaresan and Haber write. Now, scientists have to find ways to make that future happen.