EFFECT OF CHANGING WEEKLY DOSE ON ACCELERATED REPOPULATION DURING FRACTIONATED-IRRADIATION OF MOUSE TONGUE MUCOSA

The effect on accelerated repopulation of fractionated radiotherapy wi th changing weekly dose was studied in mouse oral mucosa. The basic co nventional treatment protocol comprised five daily fractions per week, 3 Gy per fraction, given over 3 weeks. In dynamic treatment schedules the same total dose was delivered by giving 5 x 2, 5 x 3 and 5 x 4 Gy , or the reverse sequence. In concomitant boost experiments five addit ional daily doses of 1 or 2 Gy respectively were applied either during the first or last week of the conventional treatment. Graded terminat ing test doses were given in order to establish dose-effect curves for complete mucosal denudation. Higher initial doses consistently result ed in longer latent times between top-up irradiation and complete epit helial denudation, corresponding to significantly higher cell densitie s at the time of test irradiation. The top-up ED(50)'S served as a mea sure of repopulation efficacy. The difference in repopulated dose betw een schedules with initial or terminal dose concentration was 10% for dynamic fractionation (p = 0.02) and about 7% in the 2-Gy boost protoc ol(p = 0.13), and no difference was observed with boost doses of 5 x 1 Gy. En conclusion, a clear though moderate increase in both cell dens ity and residual tissue tolerance can be achieved by protocols with su bstantially higher doses in the first treatment week.