If every woman aged between 50 and 79 got a mammogram every year, it would reduce deaths from breast cancer by 37 percent, according to a new statistical tool described on Sunday.

Screening women in this age group every two years would reduce mortality by 30 percent, Sandra Lee and colleagues at Harvard Medical School and the Dana Farber Cancer Institute in Boston calculated.

They hope to develop their new program into an Internet Web site that women could visit to calculate their own individual risk of breast cancer and decide when to have mammograms.

“Health policy makers can use this information to come up with public screening (recommendations),” Lee told a news conference at the annual meeting of the American Association for the Advancement of Science.

“An individual woman can use this to decide what is better for her. A 7 percent reduction may not be so big for some women. It may be important for other women.”

The American Cancer Society recommends that women 40 and older have a mammogram every year. The National Cancer Institute also recommends starting at 40 and having one every one or two years.

But the British National Health Service offers mammograms only after 50 and at three-year intervals, while other European countries often offer them every two years.

It is difficult to compute the benefits of mammograms, said Lee, because it would be unethical to design a study in which half the women were denied mammograms for decades. So she and colleague Marvin Zelen came up with a statistical calculator based on various studies of breast cancer and mammograms.

“It’s clear that the more mammograms you give, the more able you are to locate disease that a person didn’t know about,” Zelen said in a statement.

But more tests can lead to detection of non-cancerous lumps that must be biopsied, costing money and anxiety.

Lee said her model was not meant to provide an absolute guide to whether more screening is better.

“Breast cancer in a woman in her 40s is more aggressive, so it would make sense to have frequent screening,” she said - even though breast cancer is more rare in this age group.

And because breast cancer is more common in women over 50, it could also be argued that frequent screening benefits this group, she said.

There are also more benefits to having mammograms than just saving lives, said Dr. Timothy Rebbeck of the University of Pennsylvania School of Medicine.

“To the degree that you can identify tumors earlier, you can be saving some costs,” he told the news conference. Women can have a smaller surgical procedure and perhaps escape the need for chemotherapy and radiation.

About 1.2 million people a year are diagnosed with breast cancer globally and the disease kills 40,000 women and men in the United States every year.