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Mammograms' impact on breast-cancer deaths is small, study says

The belief that breast screening is crucial in cancer mortality is deeply ingrained.

D. Clarke Evans/NBAE/D. Clarke Evans/NBAE

Routine mammograms have a relatively small impact on reducing the number of women who die from breast cancer, according to a study published Thursday in one of the world's most respected medical journals.

The findings add more fire to the heated debate over the benefits of breast screening, one that continues to divide medical experts and confuse many female patients.

The study, conducted by researchers in Norway and published in the New England Journal of Medicine, found that while death rates from breast cancer have dropped substantially in the past few decades, screening programs only account for about one-third of the reduction.

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Researchers used statistical and cancer data and records to look at four groups of women in Norway, aged 50 and 69, to get their results. One group of women participated in a universal breast-screening program that was rolled out in stages across the country from 1996 to 2005. Those women were compared with a similar group of women living in the same area 10 years earlier, before Norway adopted universal screening for breast cancer.

They also looked at a group of women who were aged 50 to 69 between 1996 and 2005 but who were living in an area that didn't yet have the screening program up and running. Researchers compared that group to another group of women living in the area 10 years earlier.

After the results were tabulated, Dr. Kalager and her colleagues found that breast-cancer death rates declined by 18 per cent among the group who did not have access to a universal screening program, compared with the group from the previous decade.

The drop is attributed to improvements in treatment and care, as well as greater awareness of the disease.

When looking at the group who had participated in universal breast screening, researchers found their death rates declined by 28 per cent compared to the group from the previous decade. While it's a major drop, it only represents an additional 10 percentage-point decline, or about one-third, when compared to women who had not been involved in a breast screening program.

"I think it's very clear that the benefit that can be attributed to screening is very, very small," said Cornelia Baines, professor emerita in the department of public health sciences at the University of Toronto, who led a 2002 study that found regular mammograms didn't affect mortality rates for Canadian women in their 40s.

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But it's even possible the 10-percentage-point mortality reduction is exaggerated, the study notes, because Norway has comprehensive breast-cancer management teams across the country and it's impossible to tell whether the reduced death rate is due to screening programs or those improvements to treatment and care.

Anthony Miller, associate director of research at the Dalla Lana School of Public Health at the University of Toronto, who has also conducted research with Dr. Baines, said he wasn't surprised by the new study's findings.

"I think we're in an era where breast-cancer treatment has improved so much that it's unlikely screening is going to make any major impact," Dr. Miller said.

Yet, the idea that mammography is a crucial tool required to prevent breast-cancer deaths is ingrained in many parts of society.

Last year, the U.S. Preventive Services Task Force, an independent panel of experts, issued new guidelines stating that women in their 40s should no longer be recommended for routine breast screening, partly because of the risk of false positives. (False positives can lead to unnecessary additional tests and anxiety for patients.) But the guidelines were widely criticized by a variety of doctors, breast-cancer advocates and survivors of the disease.

In Canada, routine screening is recommended for women starting at age 50.

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Jacques Lévesque, vice-president of the Canadian Association of Radiologists and a practising radiologist in Quebec City, said he believes mammograms are vital and that research has shown they have a strong benefit.

The key to their success is how they are organized and ensuring that radiologists are well-trained, he said. "In my mind there is no doubt that screening is viable."

But Dr. Baines, Dr. Miller and other experts question whether Canada places too much emphasis on mammograms. Dr. Miller said there needs to be a greater focus on other aspects of breast health, such as monitoring for changes and bringing concerns to doctors.

"I'd tell [women]that one of the most important things that they have to do is to be aware of their own breasts," Dr. Miller said.

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About the Author

Carly Weeks has been a journalist with The Globe and Mail since 2007.  She has reported on everything from federal politics to the high levels of sodium in the Canadian diet. More

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