A new study questions the value of mammograms for breast cancer screening. It concludes that a woman is more likely to be diagnosed with a small tumor that is not destined to grow than she is to have a true problem spotted early. The work could further shift the balance of whether screening's harms outweigh its benefits. Screening is only worthwhile if it finds cancers that would kill, and if treating them early improves survival versus treating when or if they ever cause symptoms. Treatment has improved so much over the years that detecting cancer early has become less important. Mammograms do catch some deadly cancers and save lives. But they also find many early cancers that are not destined to grow or spread and become a health threat. There is no good way to tell which ones will, so many women get treatments they don't really need. It's a twin problem: overdiagnosis and overtreatment. Women were considerably more likely to have tumors that were overdiagnosed than to have earlier detection of a tumor that was destined to become large," the authors write. Dr. Joann Elmore of the University of Washington School of Medicine in Seattle, writes in a commentary in the journal that it's time to pay more attention to the "collateral damage" of screening - overdiagnosis. "The mantras, 'All cancers are life-threatening' and 'When in doubt, cut it out,' require revision," she wrote.