Dr. Archie Bleyer of St. Charles Health System in Bend, Ore., and Dr. H. Gilbert Welch of The Dartmouth Institute for Health Policy & Clinical Practice in Lebanon, N.H., said nearly a third of all newly diagnosed breast cancers have been overdiagnosed and screening, at best, is having only a small effect on the mortality rate from breast cancer.
Although fewer women are dying from breast cancer, the researchers said this was largely due to better treatment, not screening.
The researchers tested the effectiveness for mammography. To reduce the rate of death from cancer there are two prerequisites: screening must advance the time of diagnosis of cancers that are destined to cause death and it must cause fewer late-stage cancers to be found since each person diagnosed earlier via the first requisite is not diagnosed later with cancer, which is more difficult to treat.
The introduction of screening mammography in the Unites States has been associated with the doubling in the number of cases of early-stage breast cancer that are detected each year -- an absolute increase of 122 cases per 100,000 women. But, the rate at which women present with late-stage cancer has decreased by 8 percent, an absolute decrease of 8 cases per 100,000 women.
If screening worked effectively, there should be one less patient diagnosed with late-stage cancer for every additional patient whose cancer was found early on. In other words, the worst cases of cancer are not decreasing, but early cancer -- which might not cause death -- is being detected and treated.
"Our study raises serious questions about the value of screening mammography. It clarifies that the benefit of mortality reduction is probably smaller, and the harm of overdiagnosis probably larger, than has been previously recognized," the researchers said in a statement.
"Although no one can say with certainty which women are overdiagnosed, there is certainty about what happens to them: they undergo surgery, radiation therapy, hormonal therapy for five years or more, chemotherapy or a combination of these treatments for abnormalities that otherwise would not have caused illness."
The study was published in the New England Journal of Medicine.