(November 21, 2012)--A big U.S. study finds that mammograms have done surprisingly little to catch deadly breast cancers before they spread.
The analysis of roughly 30 years of data found that breast cancer screening has been spotting many early cancers that aren't life threatening.
Researchers estimate that more than a million women have been treated for cancers when they didn't need to be.
Even so, doctors say women should not give up on mammograms because they do catch some dangerous cancers.
The research was released online Wednesday in the New England Journal of Medicine.