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Study: Screening Mammography Has Little Effect on Breast Cancer Mortality

Breast cancer over-diagnosed in more than a million women, researchers say (Nov. 21)

Researchers at the Dartmouth Institute for Health Policy & Clinical Practice in Lebanon, N.H., estimate that, during the last three decades, more than a million women have been over-diagnosed with breast cancer. And despite all the screenings with mammograms, there has been no change in the incidence of metastatic breast cancer and little decrease in the rate that late-stage cancer is found, they say.

In a study published in the November 21 issue of the New England Journal of Medicine, the researchers report that nearly a third of all newly diagnosed breast cancers have been over-diagnosed and that screening, at best, is having only a small effect on the mortality rate from the disease.

The good news is that fewer women are dying from breast cancer, but the researchers concluded that this is due largely to better treatment, not screening.

The new study takes a “different view,” the researchers said, in that it uses national data over a period of three decades and details what has actually happened since the introduction of screening mammography. “It does not involve a select group of patients, a specific protocol, or a single point in time,” they said.

The researchers were interested in learning how well mammography was working in terms of basic screening principles. To reduce the rate of death from cancer, there are two prerequisites: First, screening must advance the time of diagnosis of cancers that are destined to cause death. Second, it must cause fewer late-stage cancers to be found, since each person diagnosed earlier via the first requisite is not diagnosed later with worse cancer.

Unless late-stage cancer is reduced, any observed improvement in cancer deaths must be due to something else, and for breast cancer that would primarily be better treatment, they said.

The introduction of screening mammography in the U.S. has doubled 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% — an absolute decrease of 8 cases per 100,000 women.

The researchers estimated that breast cancer was over-diagnosed in 1.3 million U.S. women in the past 30 years. In 2008 alone, they estimated that breast cancer was over-diagnosed in more than 70,000 women, accounting for 31% of all breast cancers diagnosed.

According to the researchers, women who are over-diagnosed with breast cancer undergo surgery, radiation therapy, hormonal therapy for 5 years or more, chemotherapy, or (usually) a combination of these treatments for abnormalities that otherwise would not have caused illness.

The researchers said that women 40 years of age and older have a choice about breast cancer screening. In their opinion, the benefit of mammography in lowering the death rate is considerably smaller than has been previously recognized, and the harm of over-diagnosis considerably larger.

Source: Dartmouth Institute for Health Policy & Clinical Practice; November 21, 2012.

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