FRACTIONATION EFFECT ON RADIATION-INDUCED GROWTH-RETARDATION OF TIBIAIN RABBITS AND RATS

A study of the sensitivity to fracrionation of the growing tibia of ra bbits and rats was conducted by comparing the growth of the treated ri ght bone to that of the untreated left side in each individual animal using radiographic measurements. The experimental endpoint was the per centage of normal growth 24 weeks after irradiation in rabbits and 14 weeks after treatment in rats. The results show clear dose-response re lationships in all experimental arms. A clear-cut fractionation effect was demonstrated in both species. The chi/beta-ratios determined by m aximum likelihood analysis according to the LQ-model with graded respo nses were 3.2 Gy (95% C.I. 1.1; 5.6) in rabbits and 6.9 Gy (5.3:8.7) i n rats, when all data were included in the calculations. when single-d ose data were excluded the chi/beta-values were -0.6 Gy ( -3.1;2.3) in rabbits and 5.0 Gy (3.5;7.0) in rats. Our data provide Further eviden ce that low doses per fraction should be used when irradiation of the epiphysis cannot be avoided in pediatric patients.