M uch of the reluctance of some medical organizations to endorse mammographic screening in women ages 40 -49 years could be explained by the previous lack of statistically significant mortality reduction for women in this age group enrolled in randomized clinical trials (RCTs). 1 Such absence of benefit had been attributed to the lower detection sensitivity of mammography among younger women due to their relatively denser breasts, potentially limiting tumor visualization. However, this explanation cannot account for any substantial difference in screening efficacy by age because the overall replacement of fibroglandular tissue by fatty tissue occurs gradually between ages 30 -70 years with no abrupt change observed at age 50 years. 2 Many younger women have fatty breasts and many older women have denser breasts. Moreover, improvement in mammographic technique over the past few decades allows early detection in younger women despite their somewhat denser breasts. 3 Although differences in the hormonal milieu could account for faster breast carcinoma growth rates prior to menopause, such differences do not represent an insurmountable challenge but rather one that may be solved by more frequent screening intervals.
The most plausible reasons for the previous absence of a statistically significant benefit for younger women in screening trials include their lower incidence of breast carcinoma, the relatively smaller number of younger women enrolled, and the longer period until benefit occurs unless they are screened at shorter intervals. For these reasons, more person-years of follow-up are necessary for younger women. None of the population-based RCTs were designed to include enough women ages 40 -49 years specifically to evaluate this age group.