A blood test may be able to save lives by finding cancers that have started to grow again after treatment, a study suggests.
Scientists at the Institute of Cancer Research in London found traces of breast cancer eight months before doctors would normally have noticed.
In the trial, the test found 12 cancers out of the 15 women who relapsed.
Experts said there was still some way to go before there was a test that could be used in hospitals.
Surgery to remove a tumour is one of the core treatments for cancer.
However, a tumour starts from a single cancerous cell. If parts of the tumour have already spread to another part of the body or the surgeon did not remove it all then the cancer can return.