Washington ? A major study shows giving heavy smokers special CT scans can detect lung cancer early enough to modestly lower their risk of death — the first clear evidence that a screening test may help fight the nation’s top cancer killer.

Now the hurdle is deciding who should get these spiral CT scans and how often, because the tests carry their own risks, including repeated radiation exposure and a lot of false alarms that trigger unnecessary repeat testing and even surgery.

“This finding has important implications for public health, with the potential to save many lives among those at greatest risk for lung cancer,” said National Cancer Institute Director Dr. Harold Varmus, who released the study results Thursday. But, “we don’t know the ideal way yet to do this screening.”

Specialists with the American Cancer Society — which hadn’t recommended the screening because of lack of evidence — planned to evaluate the findings when the full data is published in a few months.

Until then, “the best advice we can give is to encourage people to have conversations with their doctors about whether lung cancer screening is right for them,” said chief medical officer Dr. Otis Brawley.

Standard chest X-ray screenings haven’t proved powerful enough to reduce lung cancer deaths, so researchers turned to spiral CTs, where a rotating scanner views the lungs at various angles to spot growths when they’re about half the size that regular X-ray can. But previous small studies have produced mixed results about whether the CTs work.

The massive National Lung Screening Trial enrolled 53,000 current or former heavy smokers with no initial symptoms of cancer to try to settle the debate. It found 20 percent fewer deaths from lung cancer among those screened with spiral CTs than among those given chest X-rays, the NCI said Thursday, a difference significant enough that it ended the study early.