Mammograms that use digital imaging to help doctors spot cancer are better than standard X-rays for young women and those with dense breasts, but others can skip the extra cost and get the old-fashioned kind, a landmark study concludes.

It involved more than 40,000 women and was the largest ever done to compare mammography screening techniques.

For most women who get mammograms – those past menopause with fatty breasts – “there’s no reason to seek digital … film is just as good,” said the lead researcher, Dr. Etta Pisano, chief of breast imaging at the University of North Carolina at Chapel Hill.

However, doctors still expect digital mammograms to become more common because of their many advantages, including that they can be stored on a computer and sent electronically whenever a woman moves or a new doctor needs to see them.

“Everything is going filmless. If there’s no disadvantage to digital mammography, I would presume over time it will replace film screening,” said Dr. Kathy Brandt, chief of breast imaging at the Mayo Clinic in Rochester, Minn., who had no role in the study.

The study involved 42,760 women in the United States and Canada and was done by the American College of Radiology Imaging Network. It was funded by the National Cancer Institute.

Results were reported Friday at a medical meeting and released online by the New England Journal of Medicine, which will publish them in its Oct. 27 edition.

Mammograms cut the risk of dying of breast cancer by up to 35 percent among women 50 and older, but only by 15 to 20 percent among younger women, who tend to have dense breasts that standard film X-rays have trouble penetrating.

Digital mammograms produce images on a computer screen, where they can be enhanced or magnified to reveal lurking signs of cancer. Doctors have hoped that this would improve mammography’s usefulness for younger women.

The study aimed to learn whether they are more accurate; that is, whether they correctly detected cancer and didn’t give too many false alarms.

Each participant received both types of mammograms. Results were reviewed independently by two radiologists, and biopsies settled whether suspicious findings were cancer. Follow-up exams were done a year later to see how many cancers had been missed the first time around.

A total of 335 breast cancers were found. Both types of mammograms missed about 30 percent of them.

Digital mammograms proved 15 percent more accurate than standard film X-rays among women under 50, in whom breast cancer is relatively uncommon. Among women with dense breasts and those not yet in menopause, digital mammograms were 11 percent and 15 percent better, respectively.

For all other women, the techniques were equivalent. Pisano said the results should help prioritize who gets the more expensive tests.

“We don’t have enough digital mammography for everyone to get it, and some women won’t benefit from it anyway,” Pisano said.

Only 8 percent of mammography equipment today is digital, largely because of cost. Medicare pays an average of $85.65 for a standard film mammogram and $135.29 for a digital one.

Robert Smith, director of screening at the American Cancer Society, said no woman should delay getting a mammogram if a digital one is not available. The society recommends annual mammograms starting at age 40. The federal government advises them every one to two years for women in their 40s and then annually starting at age 50.

As for the study results, “we’d always like a newer technology to do better in every way,” but improving mammograms for certain groups of women is still a gain, he said.

In the study, about two-thirds of women either had dense breasts, were under 50 or had not reached menopause. That led Dr. Daniel Sullivan, who heads cancer imaging at the cancer institute, to conclude “it may not be completely representative of the general population” because most mammograms are done on women over 50 who have fatty breasts.

Most women under 50 have dense breasts but only one-fourth of those over 50 do, said Edward Hendrick, director of breast imaging research at Northwestern Memorial Hospital in Chicago and one of the study’s authors.

Digital machines sell for $400,000 to $500,000 compared with $75,000 to $125,000 for the standard equipment, said Vince Polkus, a products manager for GE Healthcare, one of four companies whose equipment was used in the study. Savings in film and storage space and the increased efficiency of the newer machines make up for some of the additional cost, he said.