Clinical Question: Does annual mammography offer advantages over biennial screening?

Setting: Population-based

Study Design: Cohort (prospective)

Synopsis: The authors used data from nine mammography registries to determine whether women who had a mammogram every two years (n = 2,440) were more likely to have advanced cancers than women who were screened annually (n = 5,400). They mined the databases to determine the rate of breast cancer detected by screening and the rate of interval breast cancers (i.e., the ones detected between screenings). Additionally, the authors determined whether the cancers were advanced (i.e., positive lymph nodes or metastases).

The women who were screened annually were significantly younger, more likely to use hormone therapy, more likely to have had previous biopsies, and more likely to have interval breast cancers detected. These kinds of differences typically do not occur in randomized trials. The authors report that adjusting for these factors did not affect the overall results. For women diagnosed with breast cancer between 40 and 49 years of age, 28 percent of those screened biennially had advanced disease compared with 21 percent of those screened annually. Otherwise, they found no differences in the rate of late-stage breast cancer in women screened annually versus biennially.

Bottom Line: In this observational study, women in their 40s were less likely to have advanced breast cancer if they were screened annually. This benefit did not occur with older women who were screened more frequently. The latter result is consistent with the most rigorous review of mammography screening trials. Given the significant baseline differences, what role might other intervening factors play in the interpretation of these data? Because many fewer women were screened every two years, did these women differ in other important, but hidden, ways from those who were screened annually? (Level of Evidence: 2b)