Few Women Truly Understand the Risks of Mammograms

"The question is no longer whether overdiagnosis occurs, but how should we react to it," said H. Gilbert Welch of the Dartmouth Institute for Health Policy and Clinical Practice, author of an accompanying editorial. "It's not an imperative to be screened; in fact, it's a close call."

Overdiagnosis refers to the detection of a non-life threatening cancer. In general, breast cancers come in three types: aggressive cancers that spread before they can be detected, cancers that spread slowly enough to be detected and treated early, and cancers that spread so slowly they pose no threat. Unfortunately, mammograms are best at detecting cancers in the latter category and not in the first two, and there is no way to distinguish the cancers from each other without watching them progress.

In the new study, researchers examined breast cancer rates in parts of Australia, Canada, Norway, Sweden and the United Kingdom both before and after national screening programs begin. If mammograms are actually saving lives, then there should have been an increase in the number of aggressive breast cancer cases being diagnosed. Yet while the rate of cancer diagnosis did increase, there was no increase in diagnosis of the dangerous kind.

The researchers estimated that for 2,000 women who receive mammograms regularly over a 10-year period, one life will be saved and 10 women will be needlessly treated for cancer — including chemotherapy, radiation and even breast removal.

"For too long, we've taken a brain-dead approach that says the best test is the one that finds the most cancers — but that's wrong," Welch said. "The best test is the one that finds the right cancers and nothing else."