THE KINETICS OF REPAIR FOR SUBLETHAL RADIATION-INDUCED DAMAGE IN THE PIG EPIDERMIS - AN INTERPRETATION BASED ON A FAST AND A SLOW COMPONENT OF REPAIR

The kinetics of repair of sublethal damage were studied for the epidermis of the pig after beta-irradiation. Total doses were given as 3 or 4 fractions with interfraction intervals that ranged from 0.08 h-8.0 h. Three different methods were used to analyse the results: (a) the incomplete repair (IR)-model; (b) theoretical curves showing changes in iso-effect doses, based on the IR-model, were compared with experimentally derived iso-effect doses; and (c) the percentage unrepaired dose was calculated using differences in the iso-effect doses (ED50) for moist desquamation for the various interfraction intervals. Although mono-exponential kinetics are assumed in the IR-model, the half time for repair (T1/2) was not unique, but increased with the increase in interfraction interval. For 3 fractions, with an interfraction interval of 0.25 h, a T1/2 value of 0.27 h (0.19-0.43 h) was obtained, while an interval of 4 h gave a significantly higher T1/2 value of 1.40 h (0.01-2.28 h). Similar results were obtained with the 4-fraction schedule. The experimentally derived iso-effect doses for short interfraction intervals were above the theoretical curve indicating a faster rate of repair than anticipated, based on the IR-model. Iso-effect doses for longer interfraction intervals were below the theoretical curve suggesting a slower rate of repair. For both the 3- and 4-fraction data the percentage unrepaired dose, as a function of the interfraction interval, was significantly better fitted by a bi-exponential equation than a monoexponential equation (p < 0.05). Two distinct components of repair were resolved suggesting a fast and a slow component of repair with T1/2 values of approximately 0.14 h and of approximately 2.7 h, respectively. Thus all three methods of analysis suggest that the repair of sublethal radiation-induced damage to pig skin are better explained by bi-exponential rather than mono-exponential kinetics.