Association of age and reproductive factors with benign breast tissue composition

Reproductive breast cancer risk factors are hypothesized to act by increasi ng exposure of the breast to endogenous estrogens, but few studies have qua ntitatively examined the association of these risk factors with breast tiss ue composition. This study is part of a case-control study of breast histol ogical characteristics and breast cancer risk, nested within the Nurses' He alth Study, a prospective study of 121,700 registered nurses. We studied 30 0 women who had not been diagnosed with breast cancer, but for whom we obta ined slides from a prior benign breast biopsy. We used a computer-assisted image analysis technique to assess the proportion of epithelial and fibrous stromal tissue on benign breast biopsy slides, excluding obvious mass lesi ons, Mean epithelial proportion was 5.3% (0.1-23%), and mean stromal propor tion,vas 58.7% (3-93%), Women with proliferative breast disease without aty pia had higher epithelial and stromal proportions than women with nonprolif erative breast disease (P < 0.001). Postmenopausal women had a lower epithe lial proportion (P = 0.01), and increasing age at biopsy was associated wit h decreasing stromal proportion among postmenopausal parous women (P = 0.00 4). Among premenopausal women, increasing years since last birth was associ ated with lower epithelial proportion (P < 0.001). Other reproductive risk factors were not independently associated with epithelial or stromal propor tion. Epithelial and stromal breast tissue were associated with different f actors with the exception of proliferative breast disease, which was associ ated with an increase in both epithelial and stromal proportion. The quanti tative measurement of epithelial and stromal proportion may be useful for m easuring changes in breast composition.