WASHINGTON – Screening smokers and ex-smokers with high-tech CT scans has been shown for the first time to significantly cut deaths from lung cancer, federal health officials said Thursday.

A long-awaited study of more than 53,000 middle-aged and elderly people who either smoke heavily or used to smoke found there were 20 percent fewer deaths among those who underwent annual screening with a scanning procedure known as a low-dose helical computed tomography, or CT, compared with those who got standard chest X-rays.

The findings were so striking that the National Cancer Institute, which sponsored the study, halted the National Lung Screening Trial early so the public and participants could be alerted.

“This finding has important implications for public health, with the potential to save many lives among those at greatest risk for lung cancer,” said NCI Director Harold Varmus. “This finding will be an important factor in subsequent efforts to protect the tens of millions of former and current smokers in this country against the lethality of lung cancer.”

Lung cancer is the nation’s leading cancer killer. It strikes more than 196,000 Americans each year and kills more than 159,000 — more than breast, colorectal, pancreatic and prostate cancers combined. Although significant advances have been made in reducing deaths from other leading cancers, lung cancer has remained stubbornly resistant, and most victims die.

The new finding marks the first good news about the disease in decades. There are an estimated 91.5 million current and former smokers in the United States, all of whom are at increased risk for lung cancer.

CT scans, which create a three-dimensional image of the lungs instead of a one-dimensional perspective captured by standard chest X-rays, can spot more tumors when they are smaller and can be more successfully removed through surgery, boosting the chances of survival.

The findings were hailed by the American Cancer Society, American Lung Association and others.

“With this positive trial result we have the opportunity to realize the greatest single reduction of cancer mortality in the history of the war on cancer,” James Mulshine, of Rush University Medical Center, said in a statement issued by the Lung Cancer Alliance.

Even though CT scans can be performed by most hospitals, experts stressed that it was too soon to issue any specific recommendations about the use of CT screening for lung cancer. More analysis was needed to identify exactly who might benefit, such as those who smoked less heavily, and how often they should be screened. Some expressed concern that the findings may lead to too many people undergoing unnecessary screening. The scans found abnormalities in about 25 percent of those screened, but most turned out to be a false alarm.

“As with any study of screening, there are also potential harms to be considered, such as potential overdiagnosis and needless surgeries,” said Otis Brawley, the American Cancer Society’s chief medical officer. “We have learned from the long-term analysis of other screening tests, such as mammography, that it is important to consider both benefit and harms associated with the test.”

Others expressed concern about doctors exaggerating the benefits. The findings come as health experts increasingly have been challenging the value of screening for a variety of health problems. In recent years, experts have questioned whether mammography for breast cancer, PSA testing for prostate cancer and Pap smears for cervical cancer were being overused, subjecting too many patients to needless tests and treatment.

“Will the public health community be able to manage the immediate consequences of this result, and ensure that there is a carefully planned rollout of screening to those individuals who will likely benefit, and with screening offered in places where follow-up care is available?” Peter Bach, of Memorial Sloan-Kettering Cancer Center in New York, wrote in an e-mail. “Or will we see an avalanche of entrepreneurial radiologists taking out full-page ads and billboards tomorrow overstating the magnitude of the benefit, and without any plan in place for how to handle whatever is found or how to select the right patients for screening?”

Previous studies had produced mixed results about the usefulness of the screening to try to catch lung cancer in its earliest, hopefully most treatable stages. It remained unclear whether the benefits outweighed the risks of radiation from the scans and the dangers, stress and anxiety from unnecessary biopsies and other treatment caused by false alarms.

Experts stressed that smoking was the leading cause of lung cancer, and the best way to fight the disease was to either never smoke or stop smoking.

“These findings should in no way distract us from continued efforts to curtail the use of tobacco, which will remain the major causative factor for lung cancer and several other diseases,” said Varmus, the NCI director.

Widescale CT screening, which costs about $300 for each exam, could be extremely costly. Currently, Medicare and most private health insurers do not cover CT scans for lung cancer screening. But officials at the Centers for Medicare & Medicaid Services said they would review the new findings.

The $250 million study, which started in 2002, involved 53,500 men and women at 33 sites across the country. Participants had to have smoked a pack a day for 30 years. The participants were randomly assigned to receive three annual screenings with either the CT scans or a standard chest X-ray.

The subjects were then followed for up to another five years to see who developed lung cancer. Those who were diagnosed received standard treatment. A total of 354 deaths from lung cancer occurred among the subjects who underwent CT scans, compared with 442 among those who got the chest X-rays — a 20.3 percent reduction in lung cancer mortality.