In a study published in the New England Journal of Medicine, researchers compared the rates of breast cancer before and after screening mammograms became widely available.
They found that instituting screening mammograms increased the diagnosis of very small breast cancers by more than 100 percent, from 112 cases per 100,000 women in 1976 to 234 per 100,000 in 2008. While tiny tumor diagnoses increased, mammograms did not reduce the rate of diagnosis of late-stage disease.
Over the 30-year period of study, the number of late-stage, larger cancers with a higher risk of mortality identified by mammography decreased by only 8 percent, from 109 to 94 cases per 100,000 women.
If mammography is a successful screening tool, it should detect the earliest cancers and, over time, this should be accompanied by at least as great a reduction of the larger, more advanced and dangerous tumors.
According to the authors of this study, Dr. A. Bleier and Dr. H.G. Welch, the fact that this did not occur implies that most of the early cancers were of no clinical significance, would not prove fatal and must have spontaneously resolved.
Based on these facts, they state that an estimated 1.3 million breast cancers were over-diagnosed during the 30-year period of this study and furthermore, that in 2008, breast cancers were over-diagnosed and unnecessarily treated in 70,000 women — or 31 percent of all new cancers diagnosed.
Because mammograms have not reduced the rate at which more advanced cancers are diagnosed, they argue that mammograms are having only a small effect on the rate of deaths from breast cancer.
The conundrum clinicians face, however, is that, to date, no one can distinguish a small and harmless cancer from a small but potentially fatal cancer.
In this report the authors note that in women over the age of 50, deaths from breast cancer during the period of this study decreased by 28 percent. Until now, reduction of deaths was believed to reflect a combination of early detection by mammograms and better and more effective treatments. Proponents of mammograms suggest that they contribute anywhere from 28 percent to 68 percent of the reduction in mortality.
Bleier and Welch suggest that most of the decrease in deaths comes from better treatment.
To cement their argument, they note that women diagnosed with breast cancer between the ages of 40 and 50 — none of whom had a mammogram during this study period — had the greatest improvement in outcomes, with a reduction in deaths from breast cancer of 42 percent. This benefit could only be ascribed to better treatment.