Research by scientists at Edinburgh University has revealed a third of breast cancers change form when they spread.
The study could lead to treatment being better targeted to patients needs.
Researchers studied 211 tumours which had spread from the breast to the lymph nodes, as happens in 30% of cases. They found that 82% of the cancers had changed from one type to another.
There are many different treatments for the disease when it has spread to the lymph glands, but scientists say it is imperative women are given the right one to maximise their chances of fighting the disease.
Many of the cancers studied had progressed in a way that meant a different treatment could be more effective.
It means that in some cases, hormone therapies such as tamoxifen which would not have worked for the original tumour could help treat the disease.
Other tumours had changed in a way which meant patients were being given ineffective treatment and were therefore experiencing side effects unnecessarily.
Lead researcher Dr Dana Faratian, from the Breakthrough Breast Cancer Research Unit at the University of Edinburgh, said: "We were surprised that such a high proportion of tumours change form when they spread beyond the breast.
"This suggests there is a need to test which type of disease a woman has in the lymph nodes, because it could radically alter the course of treatment she receives. We now need a clinical trial to see how these results could benefit patients."
Professor David Harrison, Director of the Edinburgh Breakthrough Breast Cancer Research Unit, said: "This research may show why some women whose cancer has spread to the lymph nodes do not respond to treatment.
"With an additional test we may be able to treat women more effectively and also make more efficient use of NHS resources."
A further clinical trial now needs to be carried out to fully evaluate the benefits of testing cancer cells in the lymph nodes. Successful trials could eventually lead to the practice being adopted by the NHS.
Last updated: 04 November 2009, 08:57
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