The question seemed simple enough: Should women in their 40s be advised to get routine mammograms in the hopes of catching breast cancers while they are still small and, presumably, easier to treat? But the more an expert panel of doctors, nurses and preventive health specialists studied the data, the harder it was to come up with an answer. Without screening, 3.5 out of every 1,000 women ages 40 to 49 will die of breast cancer in the next 10 years; regular mammography can reduce that number to 3. The panel calculated that to save one life among women in this age group, 1,900 women must be screened annually for 10 years. The other 1,899 women will receive no benefit from mammography over that period.