WASHINGTON (Reuters) – Researchers have found a way to test blood for the cells that spread cancer and said they might be able to use the method to predict whose cancer will come back after treatment.

The team at Harvard Medical School and Massachusetts General Hospital used a grant from a non-profit group to develop the test, which they tried out on samples from 20 men with prostate cancer.

They found circulating tumour cells in patients with tumours that had not spread, low-grade cancers and in patients who had their prostate glands taken out three months before.

"These are patient groups in whom we would normally not expect to see circulating tumour cells, so it gives us a tremendous amount of information about their risk," said Harvard's Sunitha Nagrath, who led the study.

Nagrath said her team's test can detect 200 circulating tumour cells from a teaspoon of blood taken from a cancer patient. Prostate cancer is the leading cancer killer of men after lung cancer.