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.

The actual difference: Of those who got a spiral CT, 354 died over the eight-year study period compared with 442 deaths among those who got chest X-rays.

But with about 200,000 new lung cancers diagnosed in the U.S. each year and 159,000 deaths, even a modest reduction could translate into big benefits. Today, lung cancer usually is diagnosed at advanced stages, and the average five-year survival rate is just 15 percent.

Still, the best advice to avoid lung cancer, stressed NCI's Varmus: Stop smoking.

Smokers and former smokers have long sought scans in the hopes of earlier lung cancer detection, even though insurance seldom covers the $300 to $400 test in people who have no symptoms.

"Clearly it saves lives," said Dr. Stephen Swensen of the Mayo Clinic, among the 33 sites that conducted the massive study. But, because it carries the burden of unnecessary tests and treatment, "society has to figure out if we can afford this."

"We want to make sure what we recommend is appropriate rather than everybody going out and asking for it," added Dr. Edward F. Patz Jr. of Duke University, who was on the committee that helped design and oversee the study.

The new trial enrolled people ages 55 to 74 who are or had been very heavy smokers, puffing at least 30 "pack-years," the equivalent of a pack a day for 30 years or two packs a day for 15 years. They had one scan a year - either spiral CT or a standard chest X-ray - for three years, and then had their health tracked.

NCI's Varmus stressed that the study provided no data on whether screening helped lighter or younger smokers.

There were risks. The CTs frequently mistake scar tissue from an old infection or some other benign lump for cancer, giving about 25 percent of the spiral CT recipients a false alarm. In an earlier Mayo Clinic study of spiral CTs, more than 70 percent had a false alarm, because that study monitored even smaller lung nodules that the newer study ignored, Swensen said.

Then there's the radiation question. The new study used low-dose spiral CTs, equivalent to the radiation from a mammogram. That's far lower than the radiation emitted by regular CT scans used to diagnose various medical conditions, but several times more than is emitted by a standard X-ray.

The NCI will analyze whether the radiation exposures from the three scans in this study changed a smoker's lifetime risk of other radiation-related cancers. Doses can be vary widely at different hospitals using different scanners, but any CTs used for screening should be low-dose, Swensen said.