Is the rate of repair of radiation-induced sublethal damage in rat spinal cord dependent on the size of dose per fraction?

In the present study the possible dependency of the kinetics of repair of sublethal damage in rat spinal cord on the fraction size has been further investigated. A wide range of sizes of dose per fraction (1.7-17.5 Gy) has been given with interfraction intervals varying from 0.5 to 24 hr. A direct method for analysis of quantal response and an incomplete-repair (IR) model for survival after fractionated exposures with short intervals were used to interpret the data. The half time of repair (T1/2) was found to be 1.6, 1.6 and 1.9 hrs for fraction sizes of approximately 4, 9, and 14 Gy respectively. There appears to be no significant effect of fraction size on the rate of repair. A clinically relevant feature observed from these experimental data is that in this tissue it takes more than 4 hrs for repair of sublethal damage, induced by a dose of approximately 4 Gy, to approach completion (i.e., sparing beyond the limit of the experimental resolution). This has to be taken into account when several fractions are to be given each day. Another feature noted from the analysis of these results is that the alpha/beta determined from the complete repair data is considerably smaller than that estimated from the incomplete repair data (interval less than or equal to 4 hrs). The nature of the inconsistency is discussed.