Repair kinetics of radiation-induced mitotic delay

The recovery from radiation-induced mitotic delay in asynchronous sarcoma-180 (S-180) ascites tumor cells has been analyzed in a manner analogous to the repair of sublethal damage. 200-R increments were separated by various fraction intervals (not exceeding the time necessary for mitosis to return to control levels) for total exposures up to 1600 R. The accumulated mitotic delay after the last exposure increment, in percent of an equivalent single exposure, decreased exponentially with overall treatment time in a bimodal fashion. An initial repair process displayed a half time of 2.3 h of overall elapsed time and was followed by a slower process with a half time of 15.1 h. Such a bimodal recovery provides an explanation of why fractionation intervals long with respect to the amitotic period resulting from a single 200 R exposure enhance mitotic delay over that of equivalent single exposures, while shorter fractionation intervals diminish it. It also predicts that mitotic delay vs. dose curves should bend toward the abscissa as the exposure time is increased with large single exposures and large fractionated exposures given over short fraction intervals.