Halftime for repair of sublethal damage in normal bladder and rectum: an analysis of clinical data from cervix brachytherapy.

The halftime for repair of sub-lethal damage is an important radiobiological parameter in analysing radiation responses and in designing new treatments involving different dose rates. This work is to resolve an inconsistency existing in the repair halftime for the bladder and rectum, two of the most dose limiting critical structures for pelvic irradiation. Both long (1.5-2 h) and short (0.3-1 h) repair halftimes have been reported previously. In this work, by reconciling clinical data from cervical brachytherapy of different dose rates and by introducing a sparing factor to consider the dose sparing occurring for critical structures, we have estimated that the most likely value of the repair halftime for bladder and rectum is short, 0.2-0.4 h if assuming alpha/beta = 2-4 Gy. The present analysis does not support the long repair halftimes reported previously for the bladder and rectum and for other normal structures.