Mammography saves lives, but may cause overdiagnosis

NEW YORK (Reuters Health) - Women who undergo mammography are less likely to die of breast cancer, but they're also more likely to be diagnosed and treated for a cancer that poses no danger to their health, according to a new review of six trials including half a million women.

This means that of every 2,000 women screened over a 10-year period, one will live longer thanks to diagnosis and treatment of breast cancer, but 10 will receive unnecessary treatment, Drs. Peter C. Gotzsche and M. Nielsen of the Nordic Cochrane Centre in Copenhagen, explain.

"It is thus not clear whether screening does more good than harm," they write. "Women invited to screening should be fully informed of the risks and benefits."

Screening mammography has been studied extensively, but its benefits and potential dangers remain controversial, the researchers point out. To investigate further, they pooled the results of six large studies in the U. S. and Europe comparing mammography to no mammography in healthy women in their 40s, 50s and 60s with no history of breast cancer. The analysis included about 500,000 women who were randomized to screening or no screening.

After seven years, they found, the women who underwent screening were 20 percent less likely to have died of breast cancer than the women in the "control" group. But Gotzsche and Nielson suggest that the reduction is likely closer to 15 percent, given the variation in quality of the trials they analyzed.

The risk that a woman would be treated for a non-life-threatening cancer, however, was 30 percent higher in the screening group.

The real differences involved are small; an individual woman would find her risk of dying from cancer reduced by 0.05 percent with regular breast cancer screening, while her risk of overdiagnosis would increase by 0.5 percent.

Advocates of screening focus on its benefits while saying little or nothing about its risks, according to the researchers. "Most women therefore tend to exaggerate substantially the benefits and to be unaware of the major harms of screening," they write.

To make screening more effective, they conclude, more research is needed on how to distinguish between life-threatening cancers and "the many benign cancers identified by screening that do not need treatment."

SOURCE: The Cochrane Library, October 18, 2006