Lack of Data for Older Women
The risk of breast cancer increases with age throughout a woman’s lifetime,  and the sensitivity and positive predictive value of screening mammography also improve as women age (Kerlikowske et al.,  1993; Mushlin et al., 1998) (Table 1-4). However, few data are available on the benefits of screening mammography in women age 70 and older,  and thus, uniform recommendations do not exist for women in this age group.

The efficacy of mammography in older women is unknown because only two randomized controlled trials included women over age 65, and the numbers were too small too provide meaningful results (Table 1-2). Furthermore, screening of some older women may be less beneficial and cost-effective because of their shortened life expectancies compared with those for women in younger age groups.

TABLE 1-4  The Probability of Developing Breast Cancer is Age Dependent

SOURCE: National Cancer Institute, Surveillance, Epidemiology, and End Results (SEER) Program, Publication 97-3536, 1997.

When a woman has serious comorbid conditions that are life limiting and would deter intervention if a tumor were discovered, screening mammography would not be helpful and is generally not recommended. However, because of the lack of data, it is difficult to determine who is likely to benefit from screening, and thus, the decision is often made on an individual basis. Recently, a retrospective cohort study among women ages 66 to 79 years suggested that some women in this age group might benefit from the continuation of screening mammography.

TABLE 1-2  Results of Randomized Clinical Trials of Screening Mammography

Results indicated an increased probability for detecting localized breast cancer in conjunction with a significantly reduced risk for detecting metastatic breast cancer among screened women (Smith-Bindman et al., 2000). A case-control study from Holland found that regular screening mammography for women ages 65 to 75 was associated with a 55 percent reduction in mortality from breast cancer,  although there was no reduction in breast cancer mortality associated with screening of women over age 75 (Van Dijck et al., 1996). Another recent study suggests that for women over age 70, screening mammography may be most beneficial and cost-effective for individuals with higher bone mineral density, a characteristic associated with a higher risk for breast cancer (Kerlikowske et al., 1999).

Challenges in Younger Women
Breast cancer screening for women under age 50 remains controversial (Lerner, 2001). Some studies have reported a survival benefit of mammography,  and many organizations advocate regular screening in this age group,  but questions have been raised as to whether the benefits outweigh the risks and costs of screening younger women (Table 1-5).

TABLE 1-5 Results of Randomized Controlled Trials for Women Ages 40 to 49 years

NOTE: RR, relative risk; CI, confidence interval.
^aInitial randomization was 1978; additional women ages 45-49 years were randomized starting in 1982.
SOURCE: adapted from Kerlikowske (1997).

As a result of the controversy, some effort has been made to develop guidelines based on risk to help women make individual decisions about when to begin screening (Gail and Rimer, 1998). It was beyond the charge of the present committee to revisit the issue of whether women under age 50 should undergo routine screening mammography,  so the discussion in this report will simply review some of the issues behind the controversy, with an emphasis on how technology development may overcome some of the difficulties associated with the screening of women in this age group.

Statistical analysis of pooled data from seven randomized clinical trials indicated that screening mammography reduced breast cancer mortality by about 16 to 18 percent in women under age 50 (Berry,  1998; Kerlikowske 1997; National Institutes of Health, 1997). However, the lag time between initiation of screening and clear demonstration of a reduced mortality was more than 10 years, whereas it was about 5 years for women over age 50. The lower absolute risk of cancer among women under age 50 implies that even if relative mortality benefits were equal for women under and over age 50, absolute risk reduction would remain considerably lower for younger women. This disparity would not be corrected by improved screening technology or adjustment of screening intervals (Sirovich and Sox, 1999). Furthermore, because breast cancer is less common among women under age 50 than among older women, more individuals need to be screened to identify a case of occult breast cancer or to prevent a death from breast cancer.

One of the difficulties with screening women in their 40s is that most such women are premenopausal and are therefore likely to have greater breast density than postmenopausal women,  whose breast tissue often (but by no means always)  contains a higher percentage of fatty tissue.