Time-dose relationships for the cure of an experimental rat tumor with fractionated radiation

Abstract

A detailed analysis has been carried out of tumor cure doses for three fractions per week X-ray treatments of an experimental rhabdomyosarcoma in rats. The tumor cure doses rise sharply between 1 and 5 days, have a slight dip at 10 days, and then rise steadily from 10 to 50 days. The 10–50 day portion of the time-dose relationship appears to be linear when plotted on a linear coordinate plot of tumor cure dose versus time; the time-dose relationship is not linear when plotted on log-log coordinates. Because a constant number of fractions/week were used in all fractionation schemes, the relative importance of time and fraction number could not be assessed directly. An analysis of the tumor cure time-dose relationship and the known in vivo and in vitro characteristics of the tumor cells suggests that there is effective reoxygenation in fractionation schemes longer than 10 days, and that repair of sublethal damage is a more important factor in the increases of the tumor cure dose with fractionation than is repopulation between treatments. A comparison of these tumor cure time-dose relationships with previously published normal tissue tolerance time-dose relationships shows that three fraction/week treatments lasting 5–33 days give no better and often worse results than single doses, but that a three fraction/week treatment lasting 50 days gives dramatically better “therapeutic results”.