Screening mammograms work, but effects are modest, study finds

Screening mammography helps reduce deaths from breast cancer, but its effects are modest, according to a new study.

Researchers used data from the Norwegian breast cancer screening program to quantify screening mammography's effect on breast cancer mortality. Norway began its breast cancer screening program in 1996 in select counties and rolled it out geographically over the next nine years, offering mammography every two years to women between 50 and 69 years of age. The staggered rollout of the screening program allowed the authors to identify and analyze four groups of women, two of which lived in counties with or without screening from 1996 through 2005 (screening group and nonscreening group) and two of which mirrored these groups from 1986 through 1995 and served as historical comparisons (historical screening group and historical nonscreening group). The primary outcome measured was the incidence-based rate of death from breast cancer. The study results appear in the Sept. 23 New England Journal of Medicine.

Overall, data from 40,075 women with breast cancer were analyzed. For women in the screening group compared with the historical screening group, breast cancer death was reduced by 7.2 deaths per 100,000 person-years (rate ratio, 0.72; 95% CI, 0.63 to 0.81; P<0.001). For women in the nonscreening group compared with the historical nonscreening group, breast cancer death was reduced by 4.8 deaths per 100,000 person-years (rate ratio, 0.82; 95% CI, 0.71 to 0.93; P<0.001). The authors calculated a relative reduction in mortality of 10% in the screening group (P=0.13), concluding that the screening-related mortality difference between the current and historical groups was 2.4 deaths per 100,000 person-years, one-third of the total reduction of 7.2 deaths. Breast cancer mortality also decreased significantly in women who were too young or too old for screening mammography, a finding the authors attributed to care from a multidisciplinary breast cancer management team established as part of the nationwide screening program.

The authors pointed out that their study's total follow-up was only 8.9 years, possibly not long enough to detect the screening mammography program's full potential benefit, and that some of the women in the nonscreening group may have had mammograms that weren't recorded, among other limitations. However, they concluded that only a third of the reduced breast cancer mortality in their study could be directly attributed to screening mammography, and that medical care provided by an interdisciplinary team may also have had an important effect. An accompanying editorial agreed, saying that the study results “help confirm that the decision about whether to undergo screening mammography is, in fact, a close call.” Given the evidence that the benefit of screening mammography is modest, the editorialist wrote, “the time has come for it to stop being used as an indicator of the quality of our health care system.”