Screening mammography reduces breast cancer mortality but is associated with higher risk for diagnosis of clinically insignificant disease, according to a meta-analysis published in Issue 4 (2006) of the Cochrane Database of Systematic Reviews.

The Danish review found that women offered screening mammograms were 15 percent less likely to die of breast cancer than women who were not offered mammograms. However, women in a screened population were also 30 percent more likely to be diagnosed and treated for a cancer that would never have posed a threat to health.

“It is likely that screening mammography reduces breast cancer mortality, but the other side of the coin is the major harm of over-diagnosis and over-treatment,” said lead author Peter Gotzsche, MD, director of the Nordic Cochrane Centre in Copenhagen, Denmark.

In the United States, women are recommended to have regular mammograms beginning at age 40 years, whereas most European countries recommend screening mammography beginning at age 50 years.

The reviewers pooled findings from six randomized controlled trials involving half a million women. Most trials enrolled women ages 45 to 64 years, although one Canadian trial included women ages 40 to 49 years.

After 7 years, women in the screening group were 20 percent less likely to have died of breast cancer compared with women in the control group. The same reduction in breast cancer mortality was seen at 13 years.

According to the reviewers, the 20-percent reduction was not fully valid because not all trials were of equal quality. In balancing results ranging from a 25 to 29 percent reduction with trials that did not find a significant difference between populations, they reached a compromise conclusion that screening was associated with not more than roughly 15 percent reduction in mortality.

William Barlow, PhD, senior biostatistician at Cancer Research and Biostatistics in Seattle, Washington, was one of the readers who considered the flaws identified by the reviewers to be, for the most part, minor issues.

“But even if the risk reduction is 15 percent, that to me is a significant reduction in breast cancer mortality,” he said. “So then the real issue becomes the trade-off between a reduction in breast cancer mortality and the adverse events caused by having detected breast cancers that may not have gone anywhere.”

The reviewers estimated that for every 2,000 women invited to get mammograms for 10 years, one woman’s life will be prolonged as a result of detecting and treating a potentially lethal cancer. According to their analysis, another 10 healthy women will be transformed into cancer patients and undergo treatment needlessly.

An additional 200 will have the anxiety-inducing experience of a false positive ? being told about a suspicious finding on a mammogram that further testing reveals to be benign.

Many of the clinically insignificant cancers were ductal carcinoma in situ. About one in five cancers picked up on a mammogram ? 60,000 cases a year in the United States ? are of this type.

Barlow does not think the fault lies with mammography: “If we were better judges of the cancers, then there wouldn’t be a problem with overdiagnosis. I don’t blame screening for that. I blame our inability to determine which cancers are life threatening and which ones are not. So the challenge is to do a better job of determining the prognostic potential of that tumor rather than necessarily blaming screening for detecting it.”