But only a few deaths would be prevented among those with the lowest risk, the US National Cancer Institute-led study found.
"Our findings show that even within the group of heavy smokers, some smokers got a lot more benefit from screening and others got much less," said senior author Hormuzd Katki, an NCI investigator.
"The ones that benefit most are the ones at the highest risk of dying from lung cancer," he said. Risk factors include lengthy smoking history, family history of lung cancer and other lung diseases.
For medical groups considering developing lung cancer screening guidelines, this study should provide a way to refine recommendations based on who benefits most, Katki said.
If lung cancer is found in its earliest stage, it is 80% curable, he noted.
The downside of screening is the potential for false-positive test results – meaning something that could be cancer is seen on a scan, but turns out not to be cancer. This can result in unnecessary invasive procedures to determine if a malignancy exists.
Results of the US National Lung Screening Trial reported last year showed a 20% reduction in lung cancer deaths among heavy smokers aged 55 to 74 years who had quit smoking for no more than 15 years. However, it wasn't clear whether this reduction was weighted by risk, so the researchers compared the number of lung cancer deaths and the number of false-positive CT screening tests among more than 26 000 people in that trial with more than 26 000 people screened with chest X-rays. CT scans can reveal abnormalities that X-rays don't.
"We found that roughly 30 to 40% of smokers in the trial had some kind of positive test, and most of those are going to be false positives," Katki said.
For people at high risk for dying from lung cancer, however, the benefit of screening outweighs the risk of a false-positive test, Katki said. The 60% in the highest-risk group accounted for 88% of the lives saved and 64% of false-positive results, the researchers concluded. Meanwhile, the 20% in the lowest-risk group accounted for only 1% of lung cancer deaths prevented, the investigators found.
The report was published in the July 18 issue of the New England Journal of Medicine.
Besides finding that the value of CT screening diminished among those with fewer risks, the study found fewer false-positive tests among those at the highest risk for dying from lung cancer than among those at the lowest risk.
Given the availability of effective but potentially expensive screening, one expert said he agrees it's important to improve risk assessment models for lung cancer.
"From a public health and societal cost point of view, it is of paramount importance to understand the effectiveness of the low-dose CT screening procedure in patients with different risks of developing lung cancer," said Dr Norman Edelman, a professor of preventive and internal medicine at Stony Brook University in Stony Brook, NY
The study shows very large differences in the benefits of screening between high-risk groups and low-risk groups, Edelman said.
"Thus it might be possible for society to save a great deal of money with little negative effect if lower-risk groups were not offered screening," he said.
A report published last year in the Journal of the American Medical Association questioned whether too many false-positive test results might outweigh the benefits of CT scans for smokers.
Those researchers also concluded that only those at the highest risk of dying from lung cancer would benefit from CT screening.
For more information on lung cancer, visit the U.S. National Cancer Institute.