We need to ponder the study from Norway last September (Effect of Screening Mammography on Breast-Cancer Mortality in Norway, Mette Kalager, M.D., et al., NEJM) that tried to isolate the benefits from screening mammography from the general improvements in treatment for breast cancer that have accrued over the past 20 years. Because Norway has a unified care system for its population, all women have benefitted from the improved treatment (surgery, radiation and chemotherapy) for breast cancer.
Norway implemented regular screening for breast cancer by mammography in 1996 and did so in a progressive roll-out fashion throughout the country (about 4 million population) that allowed for detailed record-keeping as to the separate effects of mammographic screening and constantly advancing treatments in improving the outcomes from breast cancer.
In brief, the study suggests that mammography accounted for about 2.4 (one-third) of the 7.2 breast cancer deaths reduced per 100,000 women per year, which equaled about a 10% reduction in overall breast cancer mortality. Certainly this is a substantial benefit. But what is the cost of that benefit, not just in money, but in potentially adverse effects of the screening process?
Here the picture gets complicated. An editorial in the NEJM by H. Gilbert Welch, M.D., M.P.H. offered the following analysis. In the United States, a 50-year-old woman’s chance of dying from breast cancer within 10 years is now about 4 in 1,000. That means that 996 of 1000 50-year-old women will not die from breast cancer in the next decade.
If mammography reduces breast cancer deaths by 10%, it saves 10% of 4 deaths or 0.4 deaths per 1,000 women per decade. That means we need to screen 2500 women (1,000/0.4) to save one woman from dying of breast cancer each decade. That is a much larger number than has been appreciated.
Moreover, there are biological costs to screening. According to Dr. Welch, about 1,000 women of 2,500 screened would have false positive mammograms (which are always disconcerting and many of those women would end up with biopsies) and at least 10–15 would be improperly treated for breast cancer. These are sobering numbers.
What is the conclusion for now? Mammography has benefits and reduces women’s death from breast cancer. We certainly support it. However, when we take into account the progressively improving treatments for breast cancer, mammography’s life-saving benefit may be more modest than we thought, and the attendant costs of repeated screening X-rays, biopsies and even occasional misdirected treatment may be more costly than we really focused on.