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Every year, women hear messages from major charities and health professionals about the importance of mammograms to help with the early detection of breast cancer.

But as more research emerges showing the potential drawbacks of mammograms, some are questioning how useful they really are.

Fresh concerns are being raised this week after a large new study in the New England Journal of Medicine found that mammograms are leading to "substantial overdiagnosis" of breast cancer. Over the last 30 years, more than one million women in the United States have been unnecessarily treated for breast cancer that would never have developed into a life-threatening disease as a result of mammograms, the researchers said. The study also found that mammograms have only resulted in a slight decrease in the number of women with advanced cases of breast cancer.

In the study, researchers examined data from 1976 to 2008 to evaluate trends in the incidences of early- and late-stage breast cancer in women age 40 and older.

The findings have struck a nerve in the breast cancer community and threaten to create further divisions between those who believe that a mammogram can be a life-saving tool and those who think it's a highly-overrated tool. The debate over mammograms isn't new; it has been raging in recent years as health regulators in Canada, the U.S. and other countries have created new guidelines about who should be getting mammograms. In the past few years, panels of scientific experts have recommended women should get routine mammograms starting at age 50, not age 40 (unless they have a family history or other specific risk factor for the disease). Many cancer advocates denounced the new guidelines. Associations representing radiologists, who sometimes receive research funding or other forms of payment from companies making imaging equipment, have even said the new rules will cost lives.

Those same groups have wasted little time casting doubt on the new research. The Washington Post reported that the American Cancer Society said the results need to be "viewed with caution" while the American College of Radiology said the research may result in "lost lives."

The heart of the issue over mammograms is how well they accurately detect cancer. In the new study, researchers found the introduction of mammograms has doubled the number of early-stage cases of breast cancer detected each year, going from 112 to 234 cases per 100,000 women. The researchers concluded that only eight of the additional 122 early-stage breast cancers detected through mammography would develop to advanced disease, meaning many women are undergoing unnecessary biopsies, surgeries, drug treatments or other interventions.

At the same time, mammograms are only linked to a slight reduction in the number of women with advanced cases of breast cancer. In 2008 alone, they estimate that 70,000 women were overdiagnosed, representing 31 per cent of all diagnosed cases of breast cancer.

More researchers are starting to believe that improvements in treatment – rather than early detection from mammograms – will help reduce the overall breast cancer mortality rate.