Breast density as a predictor of mammographic detection: Comparison of interval- and screen-detected cancers

Background: Screening mammography is the best method to reduce mortality fr om breast cancer, yet some breast cancers cannot be detected by mammography , Cancers diagnosed after a negative mammogram are known as interval cancer s. This study investigated whether mammographic breast density is related t o the risk of interval cancer. Methods: Subjects were selected from women p articipating in mammographic screening from 1988 through 1993 in a large he alth maintenance organization based in Seattle, WA, Women were eligible for the study if they had been diagnosed with a first primary invasive breast cancer within 24 months of a screening mammogram and before a subsequent on e. Interval cancer case subjects (n = 149) were women whose breast cancer o ccurred after a negative or benign mammographic assessment. Screen-detected control subjects (n = 388) were diagnosed after a positive screening mammo gram, One radiologist, who was blinded to cancer status, assessed breast de nsity by use of the American College of Radiology Breast Imaging Reporting and Data System. Results: Mammographic sensitivity (i.e., the ability of ma mmography to detect a cancer) was 80% among women with predominantly fatty breasts but just 30% in women with extremely dense breasts. The odds ratio (OR) for interval cancer among women with extremely dense breasts was 6.14 (95% confidence interval [CI] = 1.95-19.4), compared with women with extrem ely fatty breasts, after adjustment for age at index mammogram, menopausal status, use of hormone replacement therapy, and body mass index. When only those interval cancer cases confirmed by retrospective review of index mamm ograms were considered, the OR increased to 9.47 (95% CI = 2.78-32.3), Conc lusion: Mammographic breast density appears to be a major risk factor for i nterval cancer.