THURSDAY, Nov. 4 (HealthDay News) -- Annual screening for lung cancer with low-dose CT scans cut mortality rates in older, current or former heavy smokers by 20 percent, a major U.S. government study finds.

Given the large numbers of Americans who fall ill from lung cancer -- the nation's leading cancer killer -- a 20 percent drop in deaths could be significant, experts noted.

And, unexpectedly, annual CT screening also cut deaths from any cause by 7 percent, according to the $250 million National Lung Screening Trial (NLST), sponsored by the U.S. National Cancer Institute (NCI).

That benefit has yet to be explained, NCI Director Dr. Harold Varmus said during a news conference Thursday.

The results, which were announced at the news conference and published in the Nov. 4 issue of Radiology, were significant enough to trigger an early halt to the trial once the scans' benefits became clear.

"Lung cancer is a major cause of death from cancer in the U.S. and in the world . . . Over 85 percent of those victims are current or former smokers," Varmus said Thursday. "These findings will be an important factor in subsequent efforts to protect tens of millions of current and former smokers in this country from the lethality of lung cancer."

The researchers are assuming that the reason for the benefit lies in the scan's ability to detect tumors earlier, when they are smaller and more treatable.

However, Dr. Michael Unger, director of the Pulmonary Cancer Detection and Prevention Program at Fox Chase Cancer Center in Philadelphia, cautioned that "researchers will have to pore over the paper and look at it for validity" before firm conclusions can be drawn.

And Varmus stressed that no concrete recommendations are being made until all the data has been published.

Lung cancer has the highest mortality rate of any cancer in the United States. According to the American Cancer Society, more than 220,500 new cases of lung cancer are diagnosed each year and the disease claims the lives of more than 157,000 people annually.

Lung cancer's high mortality is largely due to the fact that there is no effective screening method "to detect lung cancer early," Unger said. Prior research to assess whether standard chest X-rays might help reduce lung cancer deaths have not shown an effect.

The new study involved 53,500 current and former heavy smokers (an average of one pack a day for at least 30 years) aged 55 to 74 who were randomized to undergo either low-dose helical CT or a chest X-ray once a year for three years.

According to a news release from the NCI, chest X-ray "produces a single image of the whole chest in which anatomic structures overlie one another," whereas helical CT produces "a multiple-image scan of the entire chest."

Deaths from lung cancer and from other causes were lower in the CT screening than the X-ray group, the researchers found. By Oct. 20, 2010, 354 people in the group receiving CT scans had died from lung cancer versus 442 deaths in those receiving chest X-rays -- a 20.3 percent drop in mortality rates.

However, there are downsides to low-dose helical CT screening, noted NCI Deputy Director Dr. Douglas Lowy. Those include radiation exposure (more than from an X-ray); cost, as most insurance carriers and Medicare do not reimburse for the CT scan; and a high rate of false-positives that could result in a lung biopsy or thoracic surgery to provide a true diagnosis -- along with the worry to patients and their loved ones.

The NCI also notes that the participants in the trial were an urban, "highly motivated" group screened at major medical centers, and the results may not be the same among other populations.

In a statement released Thursday, Dr. Bruce Johnson, spokesman for the American Society of Clinical Oncology (ASCO), addressed the issue of expense.

"What has happened here is that the technology shows you can cut down on lung cancer deaths, the leading cause of cancer mortality, and save nearly as many lives as the number of people who die from breast cancer per year," he said. "We as a medical community now need to figure out how to do this in a way that the cost is acceptable to the public."

The findings also do not shed light on whether CT screening would be effective in other groups of people, such as light smokers, nonsmokers or younger people, said Richard Fagerstrom, one of the trial investigators.

"The application of these methods to a more general population may be difficult to decipher," Varmus added. "We don't yet have a recommendation for the scheduling of screening or for how many years it should be used. That will have to be left to others once the full dataset is available."

Still, advocates for lung cancer patients expressed optimism.

"Now we have validation from the NCI that screening people at high risk for lung cancer with CT scans can significantly reduce mortality in lung cancer, which is causing more deaths each year than breast, prostate, colon and pancreatic cancers combined," Laurie Fenton-Ambrose, president and CEO of Lung Cancer Alliance, said in a statement.

The larger public health message on lung cancer prevention should not be forgotten, however, Varmus added.

"No one should come away from this announcement believing that it's safe to continue to smoke or start smoking," he said. "Not smoking and quitting smoking remain the best defense against lung cancer."

More information

There's more on the study at the U.S. National Cancer Institute.