Rapid repair of sublethal damage in irradiated cells

Several models have evolved in recent years which assume that the time dependent reduction in killing efficiency observed between multiple exposures results from repair of sublethal damage. Our accumulation of damage model incorporates this view and may be applied to the interpretation of either split-dose or dose-rate phenomena. We have analyzed split-dose data and demonstrated that at least two independent processes can function concurrently in the algal eukaryote Chlamydomonas reinhardi irradiated with 1.5 MeV electrons at dose rates up to 100,000 rad/sec. When irradiations were made at conventional dose rates, simple exponential repair as a function of time is observed, indicating a repair process which follows first order kinetics. However, when irradiation times are sufficiently reduced and cells maintained at temperatures above 25/sup 0/C, survival reflects the sum of two exponential components. These two components reflect two repair processes which may be separated by a technique analogous to that used for separating decay curves for mixed radioisotopes having different half-lifes. When split-dose data from experiments between 25/sup 0/C and 40/sup 0/C are handled in this manner, we find that the slower process has a characteristic rapair time on the order of 25 minutes, while that of the faster process ismore » about 2 minutes.In addition the data indicate that each process repairs a different type of damage. This being the case, one of these two distinct types of damage is not observable using conventional techniques but can be found only when very high dose rates and short interfraction intervals are used. Both types of damage must be considered when analyzing cell survival data in terms of energy deposition patterns and LET since it is possible that the site sizes and interaction distances for the two types of damage may differ.« less