Comparison of cell proliferation kinetics in human and experimental tumors: response to irradiation

Available data on the parameters of cell kinetics in human solid tumors were reviewed. The mean duration of the cell cycle is 2 days; for G1 it is 1 day, for S it is 18 hours, and for G2 it is 6 hours. G1 has the greatest variability from tumor to tumor. The cell cycle and its phases are shorter in animal tumors but the relative durations and variabilities are similar. The labeling index (LI) and the growth fraction vary widely among human tumors, but they are correlated with the histologic type of the tumor. There is a correlation between the mean doubling time and the mean LI of the various histologic types of tumors. Cell loss is considerable in all human tumors, but it is more important in tumors with a high LI. Metastases and recurrence appear to have shorter doubling and cell cycle times than the primary tumor. There seems to be in patients with metastases a correlation between the survival and the growth rate of the metastases. There does not seem to be any correlation between the LI and the probability of dissemination. The depression of LI after irradiation is more important in human tumors than in experimental tumors and varies widely among human tumors. After irradiation, the depopulation seems more important in tumors with a high LI. The histologic groups of tumors with a high mean LI are on the average more radiosensitive and more sensitive to drugs.