Evidence for adaptive response and implication in pulse-simulated low-dose-rate radiotherapy

Purpose: Pulsed-dose-rate (PDR) brachytherapy as a substitute for continuou s low-dose-rate (LDR) brachytherapy has a number of clinical advantages. Ho wever, early results show that some cells can exhibit an adaptive response to radiation and in PDR where many pulses are given such an adaptive respon se may play an important role in the outcome. Methods and Materials: Nine human cell lines (two normal fibroblast and sev en tumor) were evaluated for an adaptive response. Cells were given either a single adapting dose before a challenge dose or given PDR sequences for w hich the average dose rate matched the LDR dose rate, Response was assessed using the colony survival assay, Results: Five of the nine cell lines showed an adapting response to single small doses of radiation. Three of these cell lines were further investigat ed for adapting response to PDR and two of the three lines (one ovarian car cinoma and one glioma) showed an adaptive response which was dependent on p ulse size and interval. Conclusion: The data show that an adaptive response can occur in human cell s and that it can vary among cell lines. In addition, PDR sequences also pr oduced an adaptive response which could have an affect on PDR therapy if su ch a response is found in tissues, (C) 2000 Elsevier Science Inc.