Norwegian researchers cast fresh doubts on the benefits of screening mammography in the Sept. 23 edition of New England Journal of Medicine (NEJM) when they reported that screening mammography accounted for a “modest” one-third of the total 10 percent reduction in breast cancer deaths realized during the staggered implementation of a nationwide screening program.

Mette Kalager, MD, of Oslo University Hospital, and colleagues were able to establish a large comparison cohort by leveraging historical data from the country’s Breast Cancer Screening Program, which was implemented gradually over a nine-year period from 1996 to 2005.

Researchers identified and compared four groups of women: an historical screening group comprised of women residing in counties that had implemented the screening program, an historical nonscreening group residing in counties that had not implemented the program and two comparison groups that mirrored the county of residence from 1986 to 1995.

The Breast Cancer Screening Program combined biannual mammograms for women between the ages of 50 and 69 years and a county-based multidisciplinary breast cancer management team.

The researchers analyzed data for 40,075 women diagnosed with breast cancer between 1986 and 2005. Among women between the ages of 50 and 69 in the screening group, the rate of death dropped from 25.3 per 100,000 person-years in the historical group to 18.1 per 100,000 person-years, a relative reduction of 28 percent.

In the nonscreening cohort, the rate of death fell from 26.0 per 100,000 person-years in the historical group to 21.2 per 100,000 person-years, a relative reduction of 18 percent.

According to researchers, 2.4 deaths per 100,000 person-years, approximately one-third, can be attributed to the screening program alone. In addition, the researchers found that the reduction in mortality among women between the ages of 70 and 84 years was comparable to the reduction in the screening group.

The investigators acknowledged that the “follow-up time of 8.9 years may be too short to show the full potential of the screening program.” However, the largest reductions were realized in the first four years of the program.

Screening mammography provides a modest reduction in the breast cancer death rate, they concluded. But the interplay of multiple factors may be more critical.

“Most important, the apparent benefit conveyed by optimized patient care may be missed unless breast cancer screening is integrated into a well-functioning healthcare system that is available to the entire population,” stated the authors.