Screening mammography helps reduce breast cancer mortality. Women offered screening mammograms are 15 percent less likely to die of breast cancer than women who are not offered mammograms, new study shows.

Mammograms can detect breast cancer tumors that are too small to be felt as a lump, theoretically identifying breast cancer at its earliest and most treatable stage.

In the United States, women are recommended to have regular mammograms beginning at age 40. While most European countries also recommend screening mammography, their guidelines advise that women begin screening at age 50.

“It is likely that screening mammography reduces breast cancer mortality, but the other side of the coin is the major harm of overdiagnosis and overtreatment,” said lead author Peter Gøtzsche, M.D., director of the Nordic Cochrane Centre in Copenhagen, Denmark.

To determine whether offering mammograms to women with no history or symptoms of breast cancer actually reduces breast cancer mortality, the reviewers pooled together findings from six randomized controlled trials involving half a million women.

Most of the trials enrolled women ages 45 to 64, although one Canadian trial included women ages 40 to 49.

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

According to the reviewers, however, this 20 percent reduction isn’t the most accurate estimate, because not all six trials were of equal quality. The four trials that they judged to be of poorest quality yielded the greatest benefit for screening mammography — a 29 percent reduction in the risk of breast cancer mortality after seven years and a 25 percent reduction after 13 years.

However, women who had mammograms are 30 percent more likely to be diagnosed and treated for a cancer that, in the absence of screening, would never have posed a threat to their health.

The reviewers estimated that for every 2,000 women who are 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. 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 noncancerous.

“Screening mammography is clearly a double-edged sword,” said Lisa Schwartz, M.D., co-director of the Veterans Administration Outcomes Group in White River Junction, Vermont.

“Regular screening will save some lives but will cause even more women to be harmed through the unnecessary diagnosis and treatment of cancers that would never have affected their health, were it not for screening.”

Many of those cancers are of a type called ductal carcinoma in situ, or DCIS. DCIS is too small to be felt as a lump and is almost always detected with mammography. About one in five cancers picked up on a mammogram — 60,000 cases a year in the United States — is of this type.

In most cases, these abnormal cells, found in the milk ducts, will never invade the surrounding breast tissue. The problem is that researchers don’t know which of these cancers will progress and which ones won’t. As a result, all women who are diagnosed with DCIS are advised to have the lesions removed.
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The review appears in the current issue of The Cochrane Library, a publication of The Cochrane Collaboration, an international organization that evaluates research in all aspects of health care.