Senior author Dr. Karla Kerlikowske of the University of California, San Francisco and the San Francisco VA Medical Center, and co-author Rebecca A. Hubbard of Group Health Research Institute and the University of Washington School of Public Health said the study involved more than 900,000 women.
The study found women in their 40s with extremely dense breasts who undergo biennial mammography were more likely to have advanced-stage and large tumors than women who undergo annual mammography -- but annual mammograms also resulted in more false positives, the researchers said.
False positives result in women without cancer being called back for more testing, including biopsies, ultrasounds and more mammograms because a mammogram appeared positive.
"Increasing age and high breast density are among the strongest risk factors for the disease," Kerlikowske said in a statement.
Kerlikowske and colleagues reported last year risk factors help women with their doctors make decisions about when to start breast cancer screening and how often it should be repeated. For instance, a family history of breast cancer raises the likelihood of developing the disease but it does not increase the chances of advanced-stage tumors or large tumors.
"These individual decisions involve evaluating the balance between the benefits of screening -- detecting cancer early -- and the potential harms, such as false positives among healthy women," Kerlikowske said. "Some people who are at higher risk of disease may be more willing than those at lower risk to accept such potential harms of screening."
"For women ages 50-74 years old with dense breasts who are cancer free, we estimated that more than half would be recalled for additional mammography at least once over the course of 10 years of annual screening," Hubbard said.
"Screening every other year decreases this risk by about a third. The risk of false-positive results is even higher for women who begin annual screening at age 40."
The findings were published in Internal Medicine.
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