The proliferative patterns of human breast cancer cells in vivo

The cell proliferation pattern in human breast cancer has been studied in eight patients in vivo using intravenously administered 3HTdR as a pulse and as a continous label. Two to 4% of the tumor cells were engaged in DNA synthesis in six patients, and 7% and 11%, respectively, in two others. From the composite percent labeled mitosis curve the estimated time for G1 + (M/2) was about 4 hours and for DNA synthesis about 24 hours. Many cells which entered the cycle spent longer intervals in DNA synthesis and in the post-DNA synthetic phase before entering mitosis. There was a wide range of intermitotic times among the tumor cells. These findings are of significance in the planning of drug therapy.