London - A smart blood test detects the spread of breast cancer almost a year earlier than scans, research shows.
It analyses DNA in the blood to show with high accuracy whether cancer will recur after treatment.
A study of 101 women in five UK hospitals showed doctors could see how breast cancer was evolving 10.7 months before tumours appeared on scans. Although at an early stage, the findings pave the way for large clinical trials.
It is one of the first in a battery of "liquid biopsies" experts believe will revolutionise cancer treatment.
The sensitive test, which works by detecting tiny cells that are shed by a tumour as it grows, allows doctors to determine whether cancer will spread through the body. Existing techniques, which involve biopsies and scans, can be done only once the tumour has already spread.
Researchers from the Institute of Cancer Research (ICR) in London and the Royal Marsden NHS Foundation Trust believe that using such tests may even enable them to prevent a relapse if they can treat the cancer in time.
Study leader Professor Nicholas Turner, of the ICR, said: "These new blood tests can work out which patients are at risk of relapse much more accurately than we have done before, identifying the earliest signs of relapse almost a year before the patient will clinically relapse. We hope that by identifying relapse much earlier we will be able to treat it much more effectively than we can do now, perhaps even prevent some people from relapsing.
"But we will now need clinical trials to assess whether we can use these blood tests to improve patient outcome. We have launched the first of these studies already, and hope to launch large studies in future."
The test, developed at the ICR, is tailored to the DNA make-up of each tumour. For the study, published in the JAMA Oncology journal, doctors analysed DNA from tumour samples collected before treatment and identified mutations that could distinguish cancer DNA from all other genetic material in the blood.
They found 165 trackable mutations. Collecting blood from participants for up to five years and combining the data with a previous study to form a bigger sample of 144 patients, they found that, three years on, the cancer had returned in 29 patients. The test detected cancer DNA in the blood of 23 of these women prior to relapse, an average of 10.7 months before symptoms.
Cancer that had spread to the brain was less commonly detected.