The kinetics of cellular inactivation by fractionated irradiation in the R1H rhabdomyosarcoma of the rat was studied in the dose range of 1.07 to 12.50 Gy per fraction. Regimens of 1, 3, 5, 7, and 10 fractions per week for several weeks were compared. The number of clonogenic tumor cells per tumor in the course of the different treatment schedules was determined using an in vitro colony assay. The results show that the proliferation of clonogenic tumor cells is decelerated in the course of a fractionated radiotherapy. The deceleration persists for several days after end of treatment, until accelerated repopulation is initiated. The fraction of tumor cells inactivated per week was only dependent on the total dose per week, that is the cellular response was the same whether the weekly dose was applied in 1,3,5,7, or 10 fractions. Thus, the fractionation regimens were considerably more effective than expected from calculations based on single-dose in situ survival curves.
