Women aged 50 to 74 years who undergo screening mammography once every 2 years have similar risk of advanced-stage disease as, and lower risk of false-positive results than, those who have the test every year, according to the findings of a large, long-term study. The results held even for women who had high breast density or who used hormone therapy.

The findings also led senior study author Karla Kerlikowske, MD, of the University of California–San Francisco (UCSF) School of Medicine, and colleagues to conclude that women aged 40 to 49 years who have extremely dense breasts should be informed that annual mammography may minimize their risk of advanced-stage disease, but that the cumulative risk of false-positive results is high.

In 2009, the United States Preventive Services Task Force recommended mammography every 2 years rather than every 1 to 2 years for women aged 50 to 74 years. As Kerlikowske’s group pointed out in their report for JAMA Internal Medicine, controversy surrounds the questions of how often a woman should undergo screening mammography and whether screening interval should vary according to risk factors beyond age.

To compare the benefits and harms of screening mammography frequencies according to age, breast density, and postmenopausal use of hormone therapy, the investigators studied data collected from 1994 through 2008 from a prospective cohort of 11,474 women with breast cancer and 922,624 women without the disease. All the women had undergone mammography at facilities participating in the National Cancer Institute’s Breast Cancer Surveillance Consortium.

The data analysis revealed the following:

  • Biennial vs annual mammography for women aged 50 to 74 years does not increase risk of advanced-stage or large-size tumors regardless of women’s breast density or use of hormone therapy. (Approximately 3% to 6% of women aged 50 to 74 years and approximately 12% to 15% of women aged 40 to 49 years have extremely dense breasts, according to a UCSF statement.)
  • For women aged 40 to 49 years with extremely dense breasts, biennial vs annual mammography is associated with increased risk of advanced-stage cancer (odds ratio [OR] 1.89) and large tumors (OR 2.39).
  • The cumulative probability of a false-positive mammography result among women undergoing annual mammography was high among those with extremely dense breasts who were aged 40 to 49 years or who used estrogen plus progesterone; the probability was lower among women aged 50 to 74 years with scattered fibroglandular densities or fatty breasts who underwent biennial or triennial mammography.

Earlier this year, Kerlikowske and others reported in Journal of the National Cancer Institute (2013;105[5]:334-341) that women aged 66 to 89 years who undergo biennial screening mammography have similar risk of advanced-stage disease as, and lower cumulative risk of a false-positive result than, women in the same age group who undergo annual screening, regardless of comorbidity.