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Common sense would dictate a "yes" answer, but research hasn't proven that yet and some small studies have suggested they don't. The federal government intends to find out once and for all, announcing today that it is launching the biggest and most ambitious study yet to learn whether the high-tech scanners will actually cut the death rate from lung cancer.

Lung cancer rates "have remained high and have not declined as other cancers have," says John Gohagan, chief of the National Cancer Institute's division of cancer prevention and co-director of the National Lung Screening Trial. Although doctors have high hopes for spiral CT scans, right now "we don't have any accepted or proven screening test for lung cancer that we believe will reduce the mortality rate."

The National Cancer Institute (NCI), with help from the American Cancer Society, will recruit 50,000 smokers and ex-smokers at 30 leading hospitals nationwide. The cancer-free volunteers will be given either CAT scans or more conventional X-rays each year for three years, and researchers will track their health for eight years.

By the study's end, they hope to learn whether CAT scans or X-rays made a better mark on early detection and, especially, death rates. From this, they'll determine whether all smokers and former smokers should have their lungs examined routinely.

Much is at stake because of the disease's toll. Were lung cancer a specific cause of death (officially, it's listed along with all other cancers), it would be the nation's third-leading killer. The disease will claim about 160,000 American lives this year -- more than cancers of the breast, prostate, colon, and ovary combined. Another 160,000 or so Americans will be diagnosed with the disease this year. The NCI says that 87 percent of these patients smoke or used to smoke, and Gohagan says there are 90 million current or former smokers in this country.

Lung cancer is so deadly because it's not usually symptom-specific until it has already done significant damage. A patient often goes to the doctor with a cough, for instance, and an X-ray or CAT scan confirms the presence of a tumor. By that time, the cancer is well established in the lung and perhaps other parts of the body.

The trial will have "considerable application for public health policy," Gohagan says, because it will tell whether CT tests should be routine.

Spiral computerized tomography has been around for about a decade. It takes X-ray images of the whole chest and assembles them into a three-dimensional model. Whereas conventional X-rays can spot a tumor between one and two centimeters wide, a CAT scan can detect tumors smaller than a centimeter.

Whether finding them at a smaller stage will cut the death rate is one aspect of the study. Another is how it affects the patient. The scans also pick up many images that are not cancers, so researchers will look into the incidence of false alarms, whether people are treated unnecessarily, and the "emotional impact of the screening process," says Dr. Denise Aberle, imaging chief at the University of California at Los Angeles and a co-director of the trial. Also, she says, the trial will tell whether such scanning affects smoking behavior.

Even if the trial shows that CT scans don't save lives, there will be one upshot, says John Seffrin, chief executive officer of the American Cancer Society. "If it doesn't work, then those addicted [to tobacco] need to now that their only choice is to quit," he says.

What To Do

Smokers and former smokers between the ages of 55 and 74 are eligible, but there are some restrictions. If you're interested in taking part in the free trial, call 1-800-4-CANCER or the local chapter of your American Cancer Society. Visit the National Cancer Institute for more details.