The researchers estimated the probability of false-positives over a decade for the women 66 to 74.
"After 10 years of screening, almost half, or 48 percent, of the women who were screened every year had at least one false-positive," Braithwaite said. "Among those screened every two years, 29 percent had at least one false-positive after 10 years."
Results were similar in the upper age group, 75 to 89, she said.
The finding that late-stage cancers were no more likely in the women screened every other year is not a surprise, said Dr. Len Lichtenfeld, deputy chief medical officer for the American Cancer Society, who was not involved in the study.
"We've known that breast cancers in older women tends to be slower growing," he said.
The society's current guidelines recommend annual mammograms for women age 40 and over, as long as they are in good health. Age alone should not be a reason to stop having the screening, according to the guidelines, but women with short life expectancies or serious health problems should discuss the pros and cons of screening with their doctor, the society suggests.
Decisions about guidelines are constantly evolving, Lichtenfeld said, as more research is published.
Judith Malmgren, an affiliate assistant professor of epidemiology at the University of Washington, in Seattle, said the new study "confirms earlier studies that biennial screening is fine for older women with fewer false-positives."
However, she said, women with a family history or those who feel uncomfortable with the longer interval should opt for annual exams.
The study was funded by a variety of grants from the U.S. National Cancer Institute and others. Braithwaite was partially funded by the Mentored Research Scholar Award from the Ame
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