Mammograms Lower Breast Cancer Death Rate, Study Finds
Mammograms contributed to between 28% and 65% of the sharp decrease in breast cancer deaths that occurred among U.S. women from 1990 to 2000, according to a study published Thursday in the New England Journal of Medicine, the New York Times reports.
The breast cancer death rate decreased 24% from 1990 to 2000, but it has been unknown whether the decline was due to increased mammography -- which can detect cancers in earlier stages -- or improved treatment or a combination of both. The Times reports that although both mammograms and new treatments had been tested in clinical trials, "researchers did not know what occurred in the real world, where women did not always follow medical advice and doctors did not always use the tests and drugs as carefully in the studies."
There has been a general consensus among scientists that the drug therapies were effective but a sharp divide over whether the benefits of mammograms outweighed the risks of false positives and unnecessary treatments, according to the Times. The study found that both mammography and treatment contributed to the decline and researchers quantified the contribution of each factor.
For the study, statisticians assembled by the National Cancer Institute used seven statistical models to analyze data on breast cancer screening and treatment from 1975 to 2000. The death rate was steady from 1975 until 1990. It then dropped from 49.7 deaths per 100,000 women ages 40 to 75 in 1990 to 38 per 100,000 women for that age group in 2000 (Kolata, New York Times, 10/27).
The analysis found that mammography alone reduced the death rate by 7% to 23%, depending on the method of statistical analysis, for an average of 15%. Post-operative treatments, including chemotherapy and hormone therapy with drugs such as tamoxifen, reduced the death rate by an average of 19% (Peres/Crewdson, Chicago Tribune, 10/27).
The combination of mammography and adjuvant treatments decreased the death rate by 12% to 21%, the study found.
Lead author Donald Berry, chair of the biostatistics department at the University of Texas M.D. Anderson Cancer Center, said, "All seven groups concluded that the decline in the rate of death from breast cancer is a combination of screening and therapy and not restricted to one or the other." He added, "Screening would have no benefit if not followed by treatment, including surgery, and treatment has the potential to be more effective if cancer is detected at earlier stages by screening" (Reuters/Boston Globe, 10/27).
Berry said, "This is the first time that a study does it right in asking the effects of screening separate from therapy." He continued, "My own view has changed a little because of the solidifying of the benefits."
Russell Harris -- professor of medicine at the University of North Carolina and a member of the U.S. Preventive Services Task Force, which issues medical practice guidelines, and PDQ, which analyzes data for NCI -- said it was previously unclear whether the benefits of mammography outweighed the risks. The risks include false positives and the possibility of treatment for tumors that would not have spread if they were left alone, he said. "The question was not whether you could make mammography work under ideal circumstance, but does it work now, in the real world, right now." He noted that mammograms had been shown beneficial in clinical trial settings in other countries, but that the effects of screenings on U.S. women were unclear. The study "makes the benefits" of U.S. screening practices "more certain," he said (New York Times, 10/27).
However, Leonard Berlin, chair of radiology at Rush North Shore Medical Center, said a 15% mortality benefit "is not insignificant -- but it's far from the sensational figures that have been bandied about in the past. We've oversold (the benefit of) mammograms" (Chicago Tribune, 10/27).
The study is available online.