More-frequent screenings don't find more serious, late-stage cancers, the researchers found. But they do boost the number of false positives and unnecessary biopsies.
That increases anxiety and wastes time and money, says Dejana Braithwaite, University of California, San Francisco professor of epidemiology and biostatistics and lead author of the study.
Results of the national study were published online in the Feb. 5 issue of the Journal of the National Cancer Institute.
Those results are consistent with a number of earlier studies that found that women 50 to 74 years old gain the most benefit and least harm from having mammograms every two years.
For the study on older women, researchers from UCSF collected data on 2,993 women with breast cancer and 137,949 without.
There was no difference in the number or severity of late-stage cancer diagnosed in women who underwent mammography every year and those who were screened every other year, researchers found.
But about half the women who were screened every year had false positives over a 10-year period while only about 30 percent of those screened every other year did.
If the estimated 19 million women between the ages of 66 and 89 underwent screening mammograms every other year instead of every year, they would be subjected to about 1.3 million fewer false-positive examinations and about 1.1 million fewer recommendations for unnecessary biopsies.
"Our motivation is to help women make more-informed choices about breast cancer screenings in older age groups," Braithwaite said in a telephone interview.
"As they get older and battle other illnesses, they need to think more carefully about whether a breast cancer screening is beneficial to them. Perhaps they should consider being screened every other year rather than every year."
The study was paid for by the National Cancer Institute, the National Cancer Institute-funded Breast Cancer Surveillance Consortium, the American Cancer Society and the Agency for Healthcare Quality and Research.