Mammographic breast density increases the risk of breast cancer four to six times and decreases the accuracy of mammography screening. Abnormalities seen on mammography in dense breast tissue increase recall rates, reduce test specificity, and may jeopardize the benefit of screening in these women. Breast density is affected by younger age, use of hormone therapy (HT), menstrual cycle phase, parity, body mass index, and familial or genetic tendencies. The impact of each of these individual factors on the accuracy of mammographic breast screening has been studied, but the interaction of these factors has not been carefully evaluated.

Carney and associates used registry data of screening mammography on women 40 to 89 years of age from seven disparate geographic regions in the United States to evaluate the individual and combined effects of these factors on the accuracy of mammography screening. Premenopausal status was defined as women who were 40 to 54 years of age who had regular menstrual periods with no HT use. Perimenopausal and postmenopausal status was defined as women who were 55 years or older, and women whose periods had stopped or who had had both ovaries removed. This latter group was subdivided into women who used HT and those who did not.

Screening mammograms were interpreted using the American College of Radiology Breast Imaging Reporting and Data System (BI-RADS). The authors defined a positive screening examination as a BI-RADS assessment code of 0 (incomplete), 4 (suspicious abnormality), or 5 (highly suggestive of malignancy). Radiographic breast density also was classified. Breast pathology outcomes included invasive or ductal carcinoma. The follow-up period was one year or the period to the next mammogram. An examination was classified as true positive when the screening was positive and a diagnosis of cancer was confirmed.

The analysis was based on information from more than 329,000 women. The rate of true-positive examinations rose with increasing age. The rate of false-negative examinations rose with increasing breast density. Women who were receiving HT had a lower mammographic sensitivity rate and a slightly higher breast cancer rate than nonusers of HT. Sensitivity and specificity were highest among older women who did not use HT.

The authors conclude that breast density and age significantly affect the accuracy of screening mammography, with the best results being seen in women who are older than 80 years. Accuracy was lowest in younger women with dense breasts. The use of HT alone does not significantly affect sensitivity but may cause increased breast density in some women, resulting in overall decreased accuracy of mammography screening. Practitioners should begin screening mammography before initiating HT, and patients should be informed of the negative impact of HT on future mammographic studies. Mammography results also should include a statement about breast density, offering women information about the accuracy of the specific examination.