Experts sometimes disagree about when regular screening with mammography should be started. Because screening identifies cancer and cancers can be fatal, people might think that screening should be started sooner (at age 40) rather than later (at age 50). However, screening has some disadvantages, and the benefits for younger women are not as clear as those for older women.

The following are some reasons for the controversy:

• Screening, particularly in younger women, detects abnormalities that may not be cancers. Finding an abnormality often results in a biopsy to determine what it is. Thus, screening results in many more breast biopsies, sometimes causing women unnecessary anxiety and expense, as well as possibly resulting in scar tissue in the breast.

• Some breast cancers, such as in situ breast cancers (cancers that have not spread), are not fatal. Some breast cancers grow slowly and would not cause death in a woman's lifetime. However, other breast cancers continue to grow and invade other tissues. How many of the cancers detected by screening would eventually be fatal is unclear. Nonetheless, all cancers are treated because currently, health care professionals do not have enough evidence to determine which ones should be treated and which ones should not be treated.

• Mammography is less accurate in younger women. Thus, screening may miss cancers, possibly including those that could be fatal. Mammography is more accurate in women over age 50 partly because, with aging, fibroglandular tissue (composed of fibrous connective tissue and glands) in the breasts tends to be replaced with fatty tissue. Abnormalities next to fatty tissue are easier to detect with a mammogram.

• Many women must be screened to save one life. When women are older, fewer women need to be screened to save a life. For women 50 and over, screening saves lives and is recommended.