Medical researchers have struggled for years to determine how often, and at what ages, women should have mammograms.

A study published today in the Annals of Internal Medicine supports the notion that screening women every other year rather than annually reduces false positives and increases only very slightly the risk that cancer will be detected a later, harder-to-treat stage.

The issue has obvious and huge personal health and public health policy implications. Mammograms are the only means we have for screening women for breast cancer; it’s generally believed that such screening can catch cancer at its earliest, and most curable, stages. Many women owe their lives to mammograms.

But that all comes at a cost: Many women are called back for additional imaging or needle biopsies after their initial mammograms are read; the vast majority of those women turn out not to have breast cancer after all. Those false-positive readings and subsequent investigations can be agonizing to worried women, and they place added burdens on health-care resources. According to the study, “After 10 years of annual screening, more than half of women will receive at least 1 false-positive recall, and 7 percent to 9 percent will receive a false-positive biopsy recommendation.”

In 2009, the U.S. Preventive Services Task Force recommended that most women get their first mammogram at age 50 and then be screened every other year.

The new study, funded by the National Cancer Institute, analyzed data for 169,456 women who had their first screening mammogram in their 40s or 50s and 4,492 women who were diagnosed with invasive breast cancer. Researchers looked at the number of times women were called back to get additional imaging that was then found to be normal and how many times women were advised to get a breast biopsy but were not ultimately diagnosed with breast cancer. Using statistical techniques, they then estimated the probability that a woman would have a false-positive result over 10 years of screening if she got more or less frequent mammography starting at younger or older ages.

Among women who got their first mammograms at age 40, the cumulative probability after 10 years of being called back for more imaging was about 61 percent for those screened annually and about 41 percent for those screened every other year. The cumulative probability of false-positive biopsy recommendation was 7 percent with annual and 4.8 percent with biennial screening.

The authors note that women should be made aware of the risks and benefits of mammography and should discuss them with their physicians. Perhaps the most useful bit of information in the study: When interpreting radiologists have access to a woman’s earlier mammograms, the odds of a false-positive recall may be cut in half.

Another study published in the Annals of Internal Medicine this afternoon found that digital and film mammography are about equally accurate in detecting breast cancer in most women but that digital mammography is better for spotting certain (estrogen receptor-negative) tumors and for finding them in women with extremely dense breast tissue, both more common among younger women.