Three major medical groups are recommending older heavy smokers undergo annual low-dose lung CT scans to check for cancer, although they note "substantial uncertainties" linger around the practice.

The revised guidelines, released Sunday in the Journal of the American Medical Association, are based on a literature review, with much of the evidence coming from the National Lung Screening Trial, which wrapped up last year and showed smokers who had annual low-dose CT scans were more likely to survive than those who just got chest X-rays.

"Low-dose computed tomography screening may benefit individuals at an increased risk for lung cancer, but uncertainty exists about the potential harms of screening and the generalizability of results," concluded the researchers, led by Dr. Peter B. Bach with Memorial Sloan-Kettering Cancer Center in New York.

In their new guidelines, the American College of Chest Physicians, the American Society of Clinical Oncology and the National Comprehensive Cancer Network recommend doctors offer annual CT lung screening to patients aged 55 to 74 who have smoked for 30 "pack-years" (20 cigarettes a day for 30 years, or the equivalent) and continue to smoke or have quit within the last 15 years.

However, the screenings would have to be offered in settings similar to those in the NLST, "with multidisciplinary coordinated care and a comprehensive process for screening, image interpretation, management of findings and evaluation and treatment of potential cancers," the researchers write.

Lung cancer is the leading cause of cancer death in the United States and the world. About 160,000 people are expected to die of it in the U.S. this year, the researchers said. Most patients are diagnosed when the disease is too advanced to really treat, and the five-year survival rate is 16 percent, they said.

In their study, the researchers did a literature review of 591 citations, and found eight randomized control trials and 13 cohort studies that met their inclusion criteria.

The strongest evidence in favor of CT screenings for smokers came from the NLST, a randomized trial which enrolled more than 53,000 patients. The trial found that after three rounds of yearly screenings with CT scans, 20 percent fewer patients died of lung cancer than those who only had chest X-rays. Those results translate, in absolute terms, to a 0.33 percent lower risk of death for those in a CT scan group. To put it differently, doctors would need to screen 310 patients for three years to stop one cancer death, the researchers said.

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