SUBLETHAL DAMAGE REPAIR TIMES FOR A LATE-RESPONDING TISSUE RELEVANT TO BRACHYTHERAPY (AND EXTERNAL-BEAM RADIOTHERAPY) - IMPLICATIONS FOR NEW BRACHYTHERAPY PROTOCOLS

Purpose: Data were analyzed from recent experiments with the end point of late rectal obstruction in rats, involving acute and various protr acted radiation exposures. Because the end point is of direct relevanc e both for brachytherapy as well as external beam radiotherapy, the go al was to estimate the linear-quadratic (LQ) parameters alpha/beta and T-1/2, which are of importance for designing improved protraction/fra ctionation schemes. Methods and Materials: The data were fit to the LQ model, both in its standard form and in a form in which two different components of sublethal damage repair-fast and slow-are assumed. The design of the experiments was such that both slow and reasonably fast sublethal damage repair components should be separately estimated, if they were contributing to a significant degree. Results: LQ parameter estimates were alpha/beta = 4.6 Gy [4.0, 5.5] and T-1/2 = 70.2 min [59 .1, 91.4]. Despite the experimental design facilitating detection of a rapid component of repair, no statistically robust evidence for a ver y fast repair component was found. Conclusions: The long estimated rep air time for a late-responding normal-tissue end point with direct rel evance to brachytherapy suggests a variety of possible brachytherapy p rotocols that may be more efficacious than continuous low dose rate ir radiation. Just as a difference in alpha/beta ratios between early- an d late-responding tissues are a central tenet in radiotherapy, so corr esponding differences in T-1/2 values have the potential to be exploit ed, particularly for brachytherapy. (C) 1998 Elsevier Science Inc.