Mammograms can detect some breast cancers when they are small, before they have spread and become incurable. Treatment for breast cancer is more effective if breast cancer is found when it is small and localized. There are differences in the benefits of mammograms according to age.

Women 50 to 69 years old: There is good evidence that mammograms decrease deaths from breast cancer in women aged 50 to 69 years if breast cancer is adequately treated after discovery by a mammogram. Studies found fewer breast cancer deaths in women aged 50 to 69 years who received routine mammograms compared to women who did not receive routine mammograms.  Among 10,000 women 50 years old, 260 will die of breast cancer in the next 20 years. If all 10,000 women get routine mammograms over the next 20 years, 37 breast cancer deaths may be prevented. This means for women aged 50 and older, mammograms prevent one breast cancer death for every 270 women screened regularly for 20 years.

Women 40 to 49: There is some evidence that mammograms may decrease deaths from breast cancer in women in their forties. In studies involving thousands of women, there were fewer deaths from breast cancer in those who got routine mammograms than in those who did not, but this benefit only appeared ten years after screening began.

Among 10,000 women 40 years old, 37 will die of breast cancer in the next 10 years. If all 10,000 women get routine mammograms over the next 10 years, 4 breast cancer deaths may be prevented. This means for women in their forties, mammograms prevent one breast cancer death for every 2,500 women screened regularly for 10 years.

References:
Salzmann P, Kerlikowske K, Phillips K. Cost-effectiveness of extending screening mammography guidelines to include women 40-49 years old. Ann Intern Med 1997;127:955-65.
Berry, D.A. Benefits and risks of screening mammography for women in their forties: a statistical appraisal. J Natl Cancer Inst 1998;90:1431-1439.