The study, of 302 patients with early-stage breast cancer, found that about 1 in 4 patients whose tumors had not yet apparently spread beyond the breast or lymph nodes already had cancer cells circulating in their blood.
These rare cells, which are shed by tumors, are thought to be cancer seeds that can take root elsewhere in the body.
What's more, the study showed that patients who had these cells in their blood were also the ones that had the poorest outcomes.
"If they have them they had roughly four times the risk of either [the cancer] recurring or dying than those who don't have the cells," says researcher Anthony Lucci, MD, a surgical oncologist at the University of Texas M.D. Anderson Cancer Center in Houston.
Overall, 15% of patients who tested positive for circulating tumor cells (CTCs) relapsed and 10% died during the study, compared to 3% and 2%, respectively, of patients who did not test positive for CTCs.
The odds that breast cancer would progress or turn fatal rose with each CTC that was detected.
Patients with two or more CTCs, for example, were more than five times likelier to see their cancer progress and eight times more likely to die than patients with no CTCs. Those with three or more CTCs were nearly seven times more likely to see their cancer progress and nearly 12 times more likely to die over the three years, on average, they were followed for the study.
The study, which is published in The Lancet Oncology, may help to explain why about 25% of patients who catch their cancer early will see it return, even after successful surgery to remove their tumors.
The study also suggests that a blood test may one day help doctors better identify and treat patients who have fast-spreading cancers.
A Useful Tool for the Future
"This study is pretty compelling evidence that this might be a good tool for oncologists to say, 'OK, because these patients have cells in circulation, their prognosis is worse,'" says Carmen Gomez-Fernandez, MD, professor of pathology and director of breast pathology services at the University of Miami's Miller School of Medicine. Gomez-Fernandez reviewed the study for WebMD but was not involved in the research.
Other experts agree.
"What it means is that we can identify those individuals quite early on who might have a decreased survival based on a simple blood test," says Justin Stebbing, MD, PhD, a surgical oncologist at Imperial College London in England. Stebbing wrote a commentary on the study but was not involved in the research.
The test used in the study is called CellSearch. Doctors use it to judge how well patients with late-stage colorectal, breast, or prostate cancers are doing on their treatments.
It costs $500 to $600 each time it's performed, and it isn't always covered by health insurance.
"Circulating tumor cell tests are not cheap, but they give us a lot of information about the future," Stebbing says.
What's less clear, he says, is how to use the test in patients who are in earlier stages of disease.
"What that information doesn't tell us is what to do. Do we treat these patients more aggressively? Do we turn to targeted therapies that focus on the circulating tumor cell pool? We don't yet know," Stebbing says. "We don't know if eradicating [circulating tumor cells] kills cancer."
Lucci agrees that it's too early to recommend the test for early-stage patients.
"We don't know if treating patients with additional therapies or different therapies will change the outcome," he says. "That's the next step."