Women should expect one false-positive mammogram each decade if they receive yearly mammograms, according to a study released online Monday in the Annals of Internal Medicine.
In 10 years of annual mammograms, more than half of women without cancer will be called back at least once for more testing, and about one in 12 will be referred for a biopsy, according to a study of Breast Cancer Surveillance Consortium data conducted by a team led by Rebecca Hubbard, PhD, an assistant investigator at Group Health Research Institute in Seattle (Ann Intern Med, October 17, 2011).
"Screening a healthy population confers both harms and benefits," Hubbard and colleagues wrote. "False-positive recalls for additional imaging after screening mammography occur for 14% of women at first screening and for 8% at subsequent examinations, causing inconvenience and anxiety for many women."
Hubbard and her team sought to understand the frequency of false-positive test results, as well as the effects of variables such as annual versus biennial screening and starting screening at age 50 compared to age 40. The team examined data from 169,456 women ages 40 to 59 at seven regions throughout the U.S.
Most women (78.9%) had their first mammogram when they were in their 40s. Median age at first screening was 42 for women who began screening in their 40s and 53 for women who began in their 50s. Among the population, 55.6% had annual screening intervals (within nine to 18 months of a prior mammogram) and 27.6% had biennial intervals (within 18 to 30 months of a prior mammogram).
Screening every other year instead of yearly lowered a woman's probability of having a false-positive exam over the course of 10 years by about a third, from 61% to 42%, according to Hubbard and colleagues. Having prior mammograms available for comparison cut the odds of false positives in half, which suggests that if women aren't returning to the same mammography facility, they should arrange in advance to have their previous mammograms sent to the new facility, according to the group.
Women in their 40s and 50s had similar risks of having a false positive during 10 years of screening, but over the course of a lifetime, starting screening at age 40 would make a woman more likely to have false positives than if she had started at age 50 -- just because of that extra decade of screening, according to the study team. And Hubbard's group found that among women who were diagnosed with cancer, those screened at a two-year interval were not significantly more likely to be diagnosed with late-stage cancer compared to those screened at a one-year interval.
