The accuracy of screening mammography is known to vary with patient age, density of breast tissue and family history of breast cancer. Hormone replacement therapy (HRT) can increase breast tissue density; therefore, a history of HRT may reduce the specificity of mammography in detecting small breast tumors. In Australia, the proportion of women 40 years and older using HRT is now believed to be more than 20 percent. As women in this age group are targeted for screening mammography programs, the accuracy of this examination in women taking HRT is of paramount importance. Kavanagh and colleagues evaluated the effect of HRT on the sensitivity and specificity of screening mammography in detecting small breast tumors.

Women 40 years and older who were participating in a two-year national breast cancer screening program in Australia were eligible for the study. The target group consisted of women 50 to 69 years of age. Program data were linked to cancer registries and other databases to identify cases of breast cancer in screened women. Information was available on more than 103,700 asymptomatic women who were first screened in 1994.

Overall, 27 percent of the women used HRT, but the proportion varied from 9.4 percent in women more than 70 years of age to nearly 40 percent of women in the target age group. Mammogram sensitivity was 12.5 percent lower in women taking HRT than in those who were not taking it. In the first year following initial screening, the estimated overall sensitivity was 91.4 percent in nonusers compared with 83.7 percent in HRT users. A similar reduction in sensitivity occurred among HRT users in the target age group. Both results were statistically significant. Specificity also was significantly lower in HRT users. Among women diagnosed with cancer during the screening period, HRT users were more likely to have a false-negative result, even after controlling for confounding effects such as age and family history. HRT users who were not diagnosed with cancer during the screening period were slightly more likely to have false-positive results. The overall risk of detection of a small breast cancer was reduced in HRT users, but not to a level that was statistically significant.

The authors conclude that the sensitivity and specificity of screening mammography is reduced in women taking HRT, most likely because this therapy increases breast tissue density. Mammography sensitivity is 10 percent lower in the first year of screening in HRT users in the target age group of 50 to 69 years. In the second year, sensitivity is reduced by 15 percent in this group. Given the increasing trend to treat women in the target age group with HRT, it is important to alert women and health care professionals to the possible effect of HRT on the accuracy of breast cancer detection with screening mammography.