And yet, 30 "pack years" is by no means an absolute yardstick, she added. Other studies have suggested CT screening might be worthwhile in people who smoked a pack of day for 20 years -- less than what was looked at in the new trial, but still a long history of smoking.

Another consideration is the risk of radiation from the scans. While the CTs used in the study emit less radiation than conventional CT scans, it's not non-existent, said Dr. Christine D. Berg, study co-investigator and acting deputy director of the Division of Cancer Prevention at the U.S. National Cancer Institute.

Radiation is a special concern for younger people who have been shown to be both more susceptible to its effects and who have many more years of life in which to develop cancer caused by radiation, she said.

"I am very reluctant to see this move into a much younger population until we have more information," Berg said.

The next step for researchers is to do mathematical modeling that will address whether the potential upside of screening outweighs the risks of screening, Berg added.

Anxiety created by "false-positive" readings is another concern, experts said. Over the course of three years, about 24 percent of the low-dose helical CT screens were positive and 7 percent of the X-rays were positive.

About 81 percent of those patients needed follow-up imaging to determine if the suspicious lesion was cancer; 2.2 percent underwent biopsy of the lung tissue and 3.3 percent needed a bronchoscopy, in which a tube is placed into the airway, researchers said.

The vast majority of scans that were initially positive turned out to be "false positives" -- 96.4 percent of the CT scans and 94.5 percent of the X-rays. False positive means the screening test spots an abnormality, but it turns out not to be cancerous. (Put another way: 3.6 percent of the CT scans that flagged an abnormality were later found
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