"The number of CTCs before treatment can identify those patients destined to live longer vs. those who will die sooner," says researcher Neal J. Meropol, MD, director of the Gastrointestinal Cancer Program at Fox Chase Cancer Center in Philadelphia.
The test also tells the doctor whether a person is responding to treatment sooner than currently used measures, he says.
"The idea is to spare patients the side effects of ineffective therapies by finding out who won't do well much sooner," Meropol tells WebMD.
CTC Levels in Colon Cancer Patients
The researchers tried out the test on 430 people with metastatic colon cancer -- that is, cancer that had spread to other parts of the body such as the liver or lungs.
Results showed that people with low CTC blood levels before treatment lived more than twice as long as those with high levels: 19 months vs. nine months.
People with high levels relapsed much sooner: five months vs. eight months.
The results held up regardless of a person's age, type of treatment, extent of disease, or overall health, Meropol says.
He adds that a high CTC level doesn't mean that a person won't respond to therapy, just that the person "might not do as well as someone else."
The researchers also looked at what happened to people with high CTC levels after they started treatment. They found that if CTC levels dropped substantially within three to five weeks, the risk of relapse or dying did as well.
On the flip side, Meropol says, "If CTCs didn't clear from the blood after a few weeks of treatment, the patient was destined to do poorly."
The findings were presented at the annual Gastrointestinal Cancers Symposium.
Robert Mayer, MD, director of the Center for Gastrointestinal Oncology at Dana-Farber Cancer Center in Boston, says other researchers have shown that high CTC levels are associated with a poor prognosis in women with metastatic breast cancer.
The new findings offer "provocative, important pilot data [on their use] in colorectal cancer," he tells WebMD.
Mayer says that before CTC screening is ready for prime time, however, it needs to be tested in larger numbers of people.