The kinetics of cellular inactivation by fractionated irradiation in Lewis lung carcinoma was studied in the dose range of 2.3 to 6.5 Gy per fraction. Regimens of one and two fractions per day for 10 days were compared. The number of clonogenic tumor cells was determined using an in vitro soft-agar colony assay. Under the experimental conditions used, the fraction of tumor cells inactivated per day was only dependent on the total dose per day, i.e., the cellular response was the same whether the daily dose was given in one or two fractions. Thus, the two fraction per day regimen was more effective than expected from calculations based on acute in vivo radiation dose-survival curves. This result could be explained if the clonogenic tumor cells were less hypoxic during the two fractions per day regimen than the one fraction per day schedule.
