Massive New England Journal Study of Mammagrophy Sure to Provoke Controversy

This three-decade study of the effectiveness of mammography to screen for breast cancer is sure to provoke controversy. Mammography has detected many breast cancers and saved lives, but, the study says, mammography over-diagnoses — that is, in many circumstances, it purports to find problems that never would have progressed to clinical breast cancer. Here is a summary of the study's results:

The introduction of screening mammography in the United States has been
associated with a doubling in the number of cases of early-stage breast
cancer that are detected each year, from 112 to 234 cases per 100,000
women — an absolute increase of 122 cases per 100,000 women.
Concomitantly, the rate at which women present with late-stage cancer
has decreased by 8%, from 102 to 94 cases per 100,000 women — an
absolute decrease of 8 cases per 100,000 women. With the assumption of a
constant underlying disease burden, only 8 of the 122 additional
early-stage cancers diagnosed were expected to progress to advanced
disease. After excluding the transient excess incidence associated with
hormone-replacement therapy and adjusting for trends in the incidence of
breast cancer among women younger than 40 years of age, we estimated
that breast cancer was overdiagnosed (i.e., tumors were detected on
screening that would never have led to clinical symptoms) in 1.3 million
U.S. women in the past 30 years. We estimated that in 2008, breast
cancer was overdiagnosed in more than 70,000 women; this accounted for
31% of all breast cancers diagnosed.

This Washington Post story is worth reading because it previews the emerging controversy. Here's an excerpt:

The routine use of mammograms has led to more than 1 million women
being unnecessarily treated for breast cancer over the past three
decades, according to the latest scientific report to cast skepticism on
the effectiveness of the test. The study, published Wednesday in the New England Journal of
Medicine, concluded that nearly one-third of women diagnosed with breast
cancer would never have developed the full-blown disease if left
untreated. Nevertheless, in such cases patients typically undergo invasive
procedures such as surgery, radiation therapy, hormonal therapy and
chemotherapy, said H. Gilbert Welch, a coauthor of the study and a
professor at the Geisel School of Medicine at Dartmouth College. “These
are major medical interventions and they’re certainly not something you
would want to undergo if you didn’t need to,” he said.

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