Women should make an informed choice about mammogram screenings, the researchers say. (AP Photo/Lansing State Journal, Becky Shink)

One in three breast cancers detected by mammogram screening programs may be treated needlessly because they are unlikely to be life-threatening, researchers said Friday.

Writing in the British Medical Journal, Scandinavian researchers who reviewed data on screening programs from five countries, including Canada, found an overdiagnosis rate of 52 per cent for all cancers, including those that have not spread, and 35 per cent for invasive breast cancer.

Overdiagnosis arises since some breast cancers grow so slowly and harmlessly that the patient dies of other causes.

Since it's impossible to tell deadly cancers from harmless ones, doctors treat all breast cancers that are identified. But treatments such as radiation therapy, surgery or drugs can also cause harmful side-effects.

"One in three breast cancers detected in a population offered organized screening is overdiagnosed," Karsten Jorgensen and Peter Gotzsche of the Nordic Cochrane Centre in Copenhagen concluded after looking at trends at least seven years before and after public screening programs began in Australia, Britain, Canada, Norway and Sweden.

"This information needs to get to women so they can make an informed choice," Jorgensen said. "There is a significant harm in making women cancer patients without good reason."

Since screening programs began about two decades ago, more cases of breast cancer have been picked up. If the screening is effective, theoretically it should identify cancer at an earlier, more treatable stage, and eventually fewer numbers of advanced cancers should be found in older women. Mammograms are usually offered for women between the ages of 50 and 69.

Study methods criticized

There is also evidence breast cancer screening saves lives, said Otis Brawley, chief medical officer for the American Cancer Society, who questioned the methodology behind Jorgensen's estimate but noted it raises legitimate scientific questions.

Jorgensen's study did not take lifestyle changes into account that are thought to increase breast cancer incidence, such as women waiting longer to have their first child.

"We have eight high-quality, randomized prospective clinical trials — the gold standard in screening methodology — that consistently show that screening using mammography and clinical breast exams reduces the risk a woman will die of breast cancer by up to 35 per cent," Brawley said.

Likewise, Cancer Research UK's blog also raised concerns about the methodology of the Scandinavian study, noting it also did not take into account use of hormone replacement therapy and was too short to offer a reliable estimate of how many tumours are overdiagnosed or simply diagnosed earlier.

"We therefore do not find the results of this study to be credible," wrote Cancer Research UK's screening expert Stephen Duffy, a professor of cancer screening at the Wolfson Institute of Preventative Medicine in London. "Women should not be put off breast screening, which saves over a thousand lives a year in the U.K."

In an editorial published in the same issue of the journal, Gilbert Welch, a professor at the Dartmouth Institute for Health Policy and Clinical Research in Vermont, said the findings highlighted how mammography is one of medicine's "close calls."

"Mammography undoubtedly helps some women but hurts others," Welch said. "No right answer exists. Instead it is a personal choice."

Last month, Statistics Canada reported that, among Canadian women aged 50-69, the likelihood they had undergone a mammogram in the previous two years was nearly twice the rate in 1990, just after systematic breast-cancer screening was introduced.

Both Statistics Canada and the Canadian Cancer Society say evidence suggests mammography helps reduce mortality from breast cancer.

With files from The Associated Press