QUANTITATIVE COMPARISONS OF CONTINUOUS AND PULSED LOW-DOSE RATE REGIMENS IN A MODEL LATE-EFFECT SYSTEM

Purpose: There is increasing interest and usage of pulsed low dose rat e (PDR) brachytherapy, in which a single source is shuttled through th e catheters of an implant, typically for about 10 min each hour. This study was designed to compare the late effects produced in various PDR regimens with those from the corresponding continuous low dose rate ( CLDR) regimens. Methods and Materials: A model late-responding system was used, namely, cataract induction in the rat lens. This system has the advantage of being highly quantifiable. The rats eyes were exposed to a total dose of 15 Gy either continuously over 24 h, or with three different PDR regimens, all with the same total dose and overall time . We addressed three questions: (a) are late effects increased when a CLDR regimen is replaced with 10-min pulses repeated every hour? (b) A re late effects increased if hourly 10-min pulses are replaced with 10 -min pulses repeated every 4 h? (c) Are late effects increased if 10-m in pulses are replaced with 100-s pulses? Results: We found that the f our regimens under test, continuous, 10-min pulses each hour, 10-min p ulses every 4 h, and 100-s pulses every hour, showed no significant di fferences in cataractogenic potential, as estimated with the Wilcoxon- Gehan test. Power tests indicated that the experimental design was ade quate to detect relatively small differences in cataractogenicity betw een regimens. Conclusions: The equality of late effects from CLDR and PDR in these experiments must imply that sublethal damage repair is qu ite slow in this model late-responding system, in agreement with trend s observed in the clinic for sublethal damage repair of late sequelae. Such trends would suggest that PDR is unlikely to produce significant ly worse late effects than the corresponding CLDR regimen, which is in agreement with early clinical data using PDR. Caution, however, is st rongly recommended.