A new study suggests women 50 and older may be able to wait as long as 2 years between mammograms to look for breast cancer.

Researchers from the Fred Hutchinson Cancer Research Center in Seattle examined the records of 7,840 women diagnosed with invasive breast cancer or ductal carcinoma in situ. They wanted to find out if waiting longer in between mammograms would cause breast tumors to be found at a later stage. More advanced tumors are more difficult to treat, and may require more extensive surgery, as well as chemotherapy and radiation.

In women over 50, going 2 years between mammograms didn't seem to make a difference. These older women were no more likely to be diagnosed with advanced cancer than women who got the scans every year, the researchers report in the Journal of the National Cancer Institute (Vol. 96, No. 24: 1832-1839).

But the same wasn't true for women in their 40s. For these women, getting a mammogram every year had a bigger benefit: Their tumors were more likely to be found at an earlier stage if they got screened every year.

The findings should be taken into account when major health groups revise their guidelines for breast cancer screening, said lead researcher Emily White, PhD.

The American Cancer Society already recommends that women in their 40s get a mammogram every year to look for breast cancer.

"We have known for many years that breast cancer grows faster in younger women compared with older women," said Robert Smith, PhD, director of screening for ACS. "Based on that evidence, the ACS modified its guidelines in 1997 to recommend annual screening to women in their 40s."

This recommendation has been controversial because breast cancer is relatively uncommon in younger women, and providing mammograms is costly. White said her study doesn't settle the question of whether women in this age group should get mammograms. But if they do choose to be screened, she said, the study shows that getting a mammogram every year is better than getting one less frequently.

"For women under 50, having screening every year compensates for the problem that they have faster growing tumors," she said. "But it doesn't compensate for the fact that they have denser breasts and it's harder to read the mammogram."

ACS also advises women 50 and older to get a mammogram every year. Does White's study mean that advice is no longer valid?

Not necessarily, said Smith. One reason ACS urges older women to get screened every year is because there are so many factors at play – physical, social, and economic -- that a more cautious approach is needed.

For example, breast density (related to how much fat is in the breast), family history of breast cancer, or use of hormone therapy all might make a difference in how fast a tumor grows and how easy it is to find.

Although there are studies that suggest it's safe to wait 2 years in between mammograms, "we cannot say exactly at what [age] women could start screening at a wider interval," Smith said. While the age of 50 may be easy to remember, it may not be the best cutoff point.

White's study will be one of many taken into consideration when it's time to review the screening guidelines, he said. At that time it may also be possible to find other studies that help define age groups or other factors that could safely allow women to wait longer between mammograms. The guidelines were last updated in 2003.

A potential advantage to screening older women less often, White said, would be the money saved. "You don't want to spend health care dollars on things that don't bring a benefit," she said.

Smith agreed that looking at the costs to society is important, but questioned how much money could really be saved.

"If women over age 50 could be screened every 2 years with the same benefit as annual screening, then the cost savings could be considerable," he said, "if we [currently] had high rates of adherence to recommended screening guidelines."

The problem is, most women don't follow the current guidelines. Right now, women are more likely to get screened every 18-36 months, Smith said. That means the cost savings from moving to a 2-year schedule might not be as large as expected, although it could still be considerable because so many women get screened.

Today, some health insurers pay for annual mammograms, and some pay only for mammograms every 2 years, Smith said. Medicare pays for women 40 and older in the program to have a mammogram every year.