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A simple blood test may be able to tell early-stage breast cancer patients if their disease is likely to recur or spread, U.S. scientists reported Tuesday.

The findings, published in the journal The Lancet, represent an important step and could help researchers develop tailored treatments based on a woman's likelihood of a cancer recurrence.

After taking blood samples from 302 women recently diagnosed with non-metastatic breast cancer, or cancer that hasn't spread, researchers found the presence of tumour cells in the bloodstream could predict future outcomes.

Women with circulating tumour cells were more likely to suffer a relapse of cancer or die from the disease in the future than patients without the cells. The greater the number of circulating tumour cells in a woman's bloodstream, the greater the chance of recurrence or death.

"It's somewhat like seeds," said Anthony Lucci, professor of surgery at the University of Texas MD Anderson Cancer Center and lead author of the study. "The higher the number of seeds that you find, the greater chance some of them could take hold and grow."

About one-quarter of women tested had circulating tumour cells in the blood. Of those, 15 per cent experienced a cancer relapse and 10 per cent died during the study period, which ran from February 2005 to December 2010. Among the group of women without circulating tumour cells in the blood, only 3 per cent experienced a relapse, while 2 per cent died during the study period. For women with three or more circulating tumour cells in their blood sample, the outcomes were much worse: 31 per cent died or experienced a relapse.

While the discovery of a link between circulating tumour cells and future cancer risk is significant, it likely won't be able to help patients in the immediate future. Scientists don't yet understand how to translate information about circulating tumour cells into treatment that could eliminate the chance of the disease coming back.

"It is a double-edged sword," Dr. Lucci said. "We have the information but we're not sure if it's just ready for use in every patient because we just don't know how to best deal with these findings."

The potential implications of the new study, such as developing a better understanding of which patients could benefit most from aggressive treatment, are hard to deny, said Alison Allan, oncology scientist and assistant director of the Pamela Greenaway-Kohlmeier Translational Breast Cancer Research Unit at London Health Sciences Centre.

"Every extra tool the clinician has to help make that decision [about treatment] means it could either save a patient from toxicity or save a patient from developing disease later," said Dr. Allan.

But important questions remain. Are circulating tumour cells resistant to chemotherapy? Why do some women with tumour cells in the bloodstream experience a cancer recurrence, while others don't? And, perhaps most important, if traditional treatments don't stop cancer from recurring or spreading in women with circulating tumour cells, what kind of treatment will?

"Despite increasing evidence supporting the use of [circulating tumour cells] as biomarkers, how this information can be integrated into present practice is uncertain," Justin Stebbing, professor in the department of surgery and cancer at Imperial College in London, wrote in a commentary published with the study.

Dr. Lucci said a major focus going forward will be understanding the biology of circulating tumour cells to determine what makes them unique, and how they may resist traditional treatments. Many scientists believe circulating tumour cells are distinct from primary cancer cells and that in order to eradicate them, targeted therapies must be developed.