Breast cancer kills 41,000 American women every year, but hundreds of thousands survive — largely because of early diagnosis. One big question is, how early? Mammograms can save lives, but can also generate "false positives" that lead to further tests and treatments that could be unnecessary. Now, after years of recommending annual mammograms starting at age 40, the American Cancer Society says 45. But other agencies disagree, leaving questions of life or death up to individual women and their doctors.