Researchers have found that a blood test may be able to save lives by finding cancers that have started to grow again after treatment.
Scientists at the Institute of Cancer Research (ICR) 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.
However, a tumor starts from a single cancerous cell. If parts of the tumor have already spread to another part of the body or the surgeon did not remove it all then the cancer can return.
Fifty-five patients who were at high risk of relapse because of the size of the tumor were followed in the study published in Science Translational Medicine.
The scientists analyzed the mutated DNA of the tumor and then continued to search the blood for those mutations.
Fifteen patients relapsed and the blood test gave advanced warning of 12 of them.
The other three patients all had cancers that had spread to the brain where the protective blood-brain barrier could have stopped the fragments of the cancer entering the bloodstream.
The test detected cancerous DNA in one patient who has not relapsed.
None of the women in the study were told that cancerous material had been detected as it would have been unethical to base decisions on such an unproven prototype.
However, the hope is that detecting cancer earlier means treatments including chemotherapy can start sooner and improve the odds of survival.
The analysis of the blood is relatively cheap. However, investigating the DNA of the tumor for mutations in the first place is still expensive.
The price is coming down as the field of cancer medicine moves from treating tumors in whichever part of the body they are discovered, towards drugs that target specific mutations in tumors.