The good news is that imaging scans do detect lung cancer early. The bad news, according to a study in the current issue of the journal Radiology, is that they are much more likely to detect harmless nodules that pose no health threat.

Results of computed tomography, or CT scans, given to 1,520 current or former smokers over five years were examined by researchers at the Mayo Clinic in Rochester, Minn. The CT scans did reveal 68 cases of lung cancer, and some cancers were discovered at an early and most curable stage.

But the scans also found nodules in more than 73% of the patients. And 96% of the nodules discovered turned out to be benign. Sixty-nine percent of the subjects had at least one false-positive result, which occurs when a surgical biopsy or observation determines the nodule is benign. The high rate of false-positive results can have expensive and potentially dangerous consequences for recipients who undergo surgery to remove them, the authors argued.

“Currently, half of lung nodules suspected of being cancerous that go to surgery outside of research study centers turn out to be benign,” said Dr. Stephen J. Swensen, professor and chairman of the radiology department at the Mayo Clinic.

And because 3% to 5% of patients undergoing lung cancer surgery die, “that is a big price to pay if it’s a benign nodule,” Dr. Swensen said.

“While there is still reason to hope that early detection of lung cancer with CT may save lives, our results lead us to be very cautious, because there’s a chance we may be doing more harm than good,” he said.