Still, earlier and more frequent detection of lung abnormalities doesn't mean more cancer diagnoses. As with other types of cancers, more frequent screening could lead to more biopsies and surgeries.

Dr. Peter Bach, director of Memorial Sloan-Kettering Cancer Center's Center for Health Policy and Outcomes, who has studied the impact of lung cancer screening, said with the new guidelines, "over-diagnosis is guaranteed." Bach said he hopes doctors will view the "B" rating as an indication that the recommendation was weak. "They are not telling people you have to do it," he said.

With heavy smokers, red flags during screening can be common, but don't always mean cancer. In as many as 40 percent of the patients, nodules can appear, but 99 percent of those abnormalities are not cancer, Cole said.

The findings of the key study on lung cancer screening are only applicable to the group of older, heavy smokers that were included, but it's possible the screening recommendations could be expanded to more groups over time as more is learned about who is at highest risk for cancer, doctors said.

Screening guidelines for other types of cancer have similarly evolved over time. The Preventive Services Task Force recommended in 2009 that routine biennial breast cancer screening be limited to women older than 50 or with other known risk factors, leaving out women in their 40s. The panel found that for those younger women, routine screening was not shown to be worth it, a stance that stirred confusion and frustration from breast cancer advocates.

That episode underscores the importance of the lung cancer screening recommendation, said Hopkins' Dennis.

"This is a group that is very conservative in their recommendations," he said. "It speaks to the power of the data, the overwhelming evidence that lung cancer screening is effective for that population. The challenge for us as a society and as a government is to find a way to pay for it and to do it."

Reuters contributed to this report.