LOS ANGELES — Advanced CT imaging can reduce deaths from lung cancer by 20 percent among heavy smokers, according to long-awaited results from the first comparison of CT scanning and regular chest X-rays, researchers said Thursday.

The results of the study among more than 53,000 former and current heavy smokers were so conclusive that the study was terminated prematurely last week and letters were sent to all the participants advising them of the results.

The findings are considered to be a major step forward in fighting lung cancer, which is expected to kill an estimated 157,000 Americans this year, because chest X-rays have never been shown to be an effective tool for identifying tumors.

“”This is the first clear demonstration that a screening procedure can be effective in reducing deaths from lung cancer,”” Dr. Douglas Lowy of the National Cancer Institute, which sponsored the $250 million study, said at a news conference.

The study “”has important implications for public health,”” added Dr. Harold Varmus, the director of the National Cancer Institute, “”but no one should believe that it is safe to continue to smoke or to start smoking. We can reduce deaths by 20 percent, but screening does not prevent lung cancer or prevent the large majority of deaths from lung cancer.””

Varmus said the scientists are still analyzing all the data and hope to prepare a scientific paper for publication within the next few months.

He also noted that the researchers are not making any recommendations about whether routine screening should be conducted in any group. Such recommendations should await completion of the analysis of the data, he said, and are usually made by other groups, such as the U.S Preventive Services Task Force.

The technology involved in the screening is called low-dose spiral CT imaging, in which a complete three-dimensional image of the chest cavity can be produced during the duration of one held breath. The technology is much more sensitive than a conventional chest X-ray, but also uses much more radiation — about the same amount associated with a conventional mammogram, according to Dr. Denise R. Aberle of the University of California, Los Angeles’ Jonsson Comprehensive Cancer Center, who participated in the study.

A typical spiral CT costs about $300 and, unless it is for diagnostic purposes, is typically not paid for by insurers.

Participants in the study were enrolled at 33 sites nationwide in 2002 and 2003. Each was between the ages of 55 and 74 at the beginning of the study and had smoked the equivalent of a pack a day for 30 years. Subjects were randomized to receive either yearly chest X-rays or yearly spiral CTs for three years, then monitored for the duration of the study. Lung cancer deaths were confirmed by a panel of experts.

By Oct. 20, a total of 354 lung cancer deaths had occurred among those in the CT arm of the study, compared to 442 in the chest X-ray group. The data-monitoring board concluded that the study had met its goal and sent a letter to Varmus on Oct. 28 recommending that the study be halted.

Despite the success, Lowy cautioned that there may be some disadvantages to screening. In addition to cost, there is the problem of false positives. About 25 percent of the CT subjects in the study had false positives. Many of those were eliminated by diagnostic CT scans, which showed the abnormalities to be scarring or other artifacts, but some required lung biopsies, which proved unnecessary.

Also, he said, “”It remains to be determined if the radiation doses increase the risk of cancer over the remainder of the participants’ lifetimes.””