Archie Bleyer, M.D., from the Oregon Health and Science University in Portland, and H. Gilbert Welch, M.D., M.P.H., from the Geisel School of Medicine at Dartmouth in Hanover, N.H., used Surveillance, Epidemiology, and End Results data (1976 through 2008) to assess trends in the incidence of early-stage breast cancer (ductal carcinoma in situ and localized disease) and late-stage breast cancer (regional and distant disease) among women aged 40 years or older.

The researchers found that the introduction of screening mammography correlated with a doubling in the number of cases of early-stage breast cancer detected each year, from 112 to 234 cases per 100,000 women. At the same time there has been an 8 percent decrease in the rate at which women are diagnosed with late-stage cancer, from 102 to 94 cases per 100,000 women. Assuming a constant underlying disease burden, only eight of the 122 additional early-stage cancers diagnosed were expected to progress to advanced disease. In the past 30 years, it is estimated that breast cancer was overdiagnosed (screening detected tumors would never have led to clinical symptoms) in 1.3 million U.S. women. In 2008, breast cancer overdiagnosis was estimated at more than 70,000 women, accounting for 31 percent of all breast cancers diagnosed.

"Our study raises serious questions about the value of screening mammography," the authors write. "It clarifies that the benefit of mortality reduction is probably smaller, and the harm of overdiagnosis probably larger, than has been previously recognized."

One author disclosed financial ties to Sigma-Tau Pharmaceuticals.

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