The test proved to be highly accurate
A genetic test can help predict the spread of breast cancer - and a woman's chances of surviving the disease.
The test, developed by scientists at Duke University Medical Center in North Carolina, should help doctors to tailor treatments more effectively to individual patients.
Doctors often decide whether a breast tumour is aggressive by searching for evidence that it has begun to spread to the lymph nodes.
In the future this will allow us to personalise treatment for each woman with breast cancer
Women whose cancer has not spread to the lymph nodes have a better chance of beating the disease than those with cancerous nodes.
But it is not a foolproof method of determining risk because
women without cancerous nodes can have a recurrence and those with many can be cured after the initial treatment.
The new technique works by analysing the activity of large clusters of 50-100 genes which have similar characteristics.
The researchers identified several clusters - known as metagenes - which appears to control how a tumour will behave, and whether it will respond to treatment.
It proved to be 90% accurate in predicting which women, who showed no or little sign that their tumours had spread to the lymph nodes, would have a recurrence of the cancer within three years.
Researcher Dr Erich Huang said: "Our model is the clearest example to date of a step toward personalised medicine.
"You don't want to tell a woman only that she resides in
risk category A or B, but rather that we can predict her
personal risk level based on her unique genomic profile."
Professor Alan Ashworth, director of the Breakthrough Breast Cancer Research Centre, said: "Once breast cancer recurs throughout the body after primary treatment the prognosis is poor and unfortunately, at present we can't predict which women this will happen to.
"This paper provides a new method, using powerful microarray technology to predict who is likely to relapse so they can be given more aggressive treatment.
"In the future this will allow us to personalise treatment for each woman with breast cancer."
The research is published in The Lancet medical journal.