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.