A new computerized version of mammography is significantly better than the conventional test at detecting breast cancer in many women, according to a major government study released yesterday.

The long-awaited study of nearly 50,000 women found that digital mammography picked up about 15 percent to 28 percent more cancers in women younger than 50, those who had not gone through menopause and those with dense breasts.

Although the test offers no advantage for other women, about half of women undergoing mammography fall into the categories that do benefit, experts said.

Based on the findings, officials at the National Cancer Institute, which sponsored the study, and the American Cancer Society recommended that women in these categories seek digital mammography where it is available.

"It's not a magic bullet, but this does find more of the cancers that kill women in these groups," said Etta D. Pisano of the University of North Carolina at Chapel Hill, who led the study. The research is being published in the Oct. 27 issue of the New England Journal of Medicine, which released the findings early to coincide with a presentation at a meeting of the American College of Radiology in Arlington. "This should save more lives."

Some other experts, however, were more cautious, saying that the advantages of the new technology, which is more expensive, appear marginal and that it is far from clear whether the approach would actually reduce the risk of dying of breast cancer.

"There does seem to be a statistical advantage, but the practical advantage is still an open question," said Donald A. Berry of the University of Texas M.D. Anderson Cancer Center in Houston.

Breast cancer strikes 211,000 U.S. women each year and kills more than 40,000, making it the leading cause of cancer and second-biggest cancer killer (after lung cancer) among women.

The death rate from breast cancer has been dropping because of improved detection and treatment. But mammograms often trigger false alarms or miss tumors.

Digital mammography works like standard mammography, except that the images are captured digitally instead of on film -- similar to how digital cameras differ from standard cameras.

The digital technology allows radiologists to manipulate the images -- by adjusting the contrast, for example -- to make them easier to interpret. That may reduce the need for follow-up tests to examine suspect areas. The approach also uses slightly less radiation, and the images are easier to store and transmit.

Until now, however, the technique had not been shown to spot more cancers. Earlier studies were relatively small and looked only at one manufacturer's system.

In the first attempt to fully evaluate the new technology, the National Cancer Institute launched the $26 million Digital Mammographic Imaging Screening Trial, involving 49,528 women at 33 sites in the United States and Canada using four different systems.

The women underwent both standard and digital mammograms, each of which was read by a different radiologist, and then had a second mammogram a year later.

Although overall there was no difference, the digital technology detected 15 percent more cancers in women with dense breasts, 21 percent more cancers in women who had not gone through menopause and 28 percent more cancers in those younger than 50, Pisano said. Conventional mammography detects only about 55 percent of cancers in such women, whose breasts tend to be denser.

"These are the hardest groups to screen," Pisano said. "And dense-breasted women tend to be at high risk for breast cancer."

Although digital mammography was no better than film-based mammography at avoiding false alarms, its ability to detect cancers is superior enough to warrant recommending it for women in the three categories, Pisano and others said.

"This digital mammography study demonstrates how new technologies are expanding our ability to detect breast cancer earlier in more women," said National Cancer Institute Director Andrew C. von Eschenbach.

Others, however, said a blanket recommendation is premature, because most breast cancers occur in women older than 50, and experts remain divided about whether screening younger women reduces their risk of dying of breast cancer.

"I think it is very misleading to tell people that digital mammography is a better alternative. We don't know that yet," said Carolina Hinestrosa of the National Breast Cancer Coalition. "Catching more cancers doesn't necessarily mean you're going to avoid more deaths from breast cancer. You just may be seeing cancer sooner, so that you know for a longer period of time you have cancer, but you aren't affecting progression."

Digital mammograms are also more expensive, costing about $135 compared with about $85 for a standard exam, she noted.

Pisano, however, said she believes digital mammography would save lives because many of the tumors it caught were the types known to pose a serious health risk.

"These are cancers that kill women and were missed on film," Pisano said. "So that suggests it does matter."

"The digital technique is showing an advantage," said Robert Smith of the American Cancer Society. "But it's not yet very widely available. . . . Women should remember that conventional mammography is a proven technology, and they should not forgo or postpone a conventional mammogram because they can't get a digital one."