More than half of healthy women who have an annual mammogram will get at least one false positive result over a 10-year period, and 7 to 9 percent will undergo a biopsy that doesn't turn out to show cancer, U.S. researchers said Monday.

Having a mammogram every other year instead of every year would cut the risk of a false positive by about a third, but it could result in catching cancers at a later stage, they reported in the Annals of Internal Medicine.

In 2009, the U.S. Preventive Services Task Force, a government-backed advisory panel, issued new guidelines that suggested women should start routine mammograms at age 50 rather than 40, in part because the tests have such high false positive rates and the benefits in lives saved did not outweigh the worry and anguish caused by the false positive results.

The change contradicted years of messages about the need for routine breast cancer screening starting at age 40 and triggered a backlash from cancer doctors and advocacy groups who said the tests save lives and are worth the risk of a false positive test result every now and then.

In the latest study, the researchers analyzed data from a large breast cancer registry that included more than 169,000 women aged 40 to 59 in seven regions around the United States.

"What we found was more than half of women participating in annual screening would have a false positive after 10 years," Rebecca Hubbard of the Group Health Center for Health Studies in Seattle, said in a telephone interview.

"The risks (of a false positive) are decreased by a third with biannual screening compared to annual after 10 years," Hubbard said.

Over the course of 10 years, screening every other year instead of yearly lowered a woman's risk of having a false positive to 42 percent from 61 percent.

And while the risk of a false positive from mammogram screening was about the same for women in their 40s or 50s, women who started screening for breast cancer in their 40s had a higher risk over their lifetime of having a false positive.

Hubbard said the findings should be used by doctors to help women make an informed decision about screening mammograms.

Hubbard said women should know that the test results have high false positive rates and the chances are pretty good that they will get called back for more tests at some point.

Knowing this could ease some of the anxiety cited by the task force as a reason not to do annual screening.

"I think it gives us quantification of risks and benefits so when individuals consider how frequently to screen they can think of what their risk of cancer is and what their risk tolerance is for potentially getting a false positive," Hubbard said.

Many groups, including the American Cancer Society, have stuck by their long-standing recommendations of a yearly breast exam for women starting at age 40, stressing that the breast X-rays have been proven to save lives by spotting tumors early, when they are most easily treated.

Hubbard and colleagues suggest women and their doctors develop a personalized breast cancer screening plan that takes into account a woman's risk for breast cancer, and her tolerance of potential false positive findings.

Breast cancer is the second-leading cause of cancer death among U.S. women, after lung cancer. It kills 500,000 people globally every year and is diagnosed in close to 1.3 million people around the world.

(Source: msnbc)