1.4 million new breast cancer cases diagnosed each year.
50,000 to 70,000 cases don't need treatment, the study said.
234 cases of early-stage cancer per 100,000 women are detected because of mammograms -- more than double the number that would otherwise be detected.
8 percent drop in late-stage cancers, from 102 to 94 cases per 100,000 women.
• A government-appointed task force calls for mammograms every other year starting at age 50 and stopping at 75.
• The American Cancer Society recommends them every year starting at age 40.