Time dependence of response of transplanted mouse mammary tumors to single or split radiation doses

The effect of single and different split radiation doses with varying rates and time intervals was investigated in an experimental system in vivo using second generation isotransplants of two mouse mammary carcinomas. The changes in the growth delay time (GDT) and tumor control rate (TCR) were analyzed, and explained by a dose dependence of the repair of sublethal radiation damage, reoxygenation and cellular repopulation in the tumors. A direct relationship was found between the size of the first of two dose fractions and the time interval between two exposures at which treatment with split doses is most effective in delaying or preventing tumor growth. Multiple exposures to unequal dose fractions are more effective than to equal fractions with the same total dose.