Cell proliferation in fibrocystic disease and postmenopause breast ducts measured by thymidine labeling.

The thymidine labeling indice (TLI) of normal and abnormal epithelium in the human breast was measured in 108 surgical specimens. The mean values for various histologic entities were: normal lobule (0.91%; normal postlobular duct 0.71%; lobular hyperplasia 0.82%; ductal hyperplasia, lobular cell type 0.74%; intraductal hyperplasia 0.79%; papillary intraductal hyperplasia 1.02%; blunt duct adenosis 0.68%; cyst 0.56%; infiltrating carcinoma 6.82%; carcinoma in situ 5.53%. The TLI of the normal lobule declined significantly with increasing age, but that of the normal postlobular duct did not. Before the menopause, the mean TLI of the normal lobule was 1.01%, and that of the normal postlobular duct was 0.75%. After the menopause the values had declined to 0.26% and 0.41%, respectively. These findings indicated that the lobules are more sensitive to stimulation by sex steroids than the postlobular ducts. The TLIs of the various benign proliferative entities were not significantly correlated with age, and like the normal lobule and postlobular duct were near the low end of the range of TLI of breast carcinoma. Benign proliferative entities with TLIs in the high part of the breast carcinoma range were not encountered.