Determination of breast tissue composition for improved accuracy in estimating radiation doses and risks in mammographic screening

Current estimates of radiation doses and risk assessments in breast screening by mammography are based on an assumption that the 'standard' female breast comprises 50% adipose tissue and 50% 'glandular' tissue by weight. This assumed composition, based principally on elemental analysis, is used to estimate the mean 'glandular' radiation dose during mammography and hence risk of the radiation exposure. More accurate information about the cellular composition of the breast could be derived from histomorphometry, but the few previous studies have not distinguished between luminal epithelium, from which most cancers arise, and the surrounding myoepithelium; nor have they allowed for the anisotropic structure of the tissue. We estimated the epithelial volume in 67 breast tissue samples from 40 women aged 38-90 years (mean age 67 years). Sections stained for luminal epithelial cytokeratins 8, 18 and 19 were subjected to image analysis. 'Glandular' tissue, comprising epithelium and surrounding stroma, constituted on average only 17.02% by volume and luminal epithelium only 0.23% by volume. We conclude that the term 'glandular' tissue is imprecise and that less than 1% of the breast volume is at radiogenic risk in mammography. A more accurate estimation of the radiogenic risk could be based on the mean dose calculated for luminal epithelium.