Tritiated thymidine labeling index of benign and malignant human breast epithelium

38 specimens of benign breast tissue from young women and 92 primary breast carcinomas were evaluated for tritiated thymidine ([3H]TdR) labeling index (TLI) by in vitro pulse labeling. The TLI on nonneoplastic, terminal breast ducts was significantly higher during the latter half than during the first half of the menstrual cycle (P less than 0.001), and a geometric mean TLI of 1.5 during the latter half of the cycle is consistent with approximately 47% turnover of these cells during the menstrual cycle. Ducts of fibroadenomas showed similar menstrual variation TLI. The range of TLI observed in the carcinomas was 0.04-18.6, arithmetic mean 3.7, geometric mean 2.1. The TLI of carcinomas was not significantly correlated with size of the tumor, but tended to be higher in women less than 50 yr old than in women older than 50 yr (P less than 0.05), and in the presence of two or more metastatic axillary lymph nodes than when fewer than two nodes were involved (P less than 0.02).