PULSE FREQUENCY IN PULSED BRACHYTHERAPY BASED ON TISSUE-REPAIR KINETICS

Purpose: Investigation of normal tissue sparing in pulsed brachytherap y (PB) relative to continuous low-dose rate irradiation (CLDR) by adju sting pulse frequency based on tissue repair characteristics. Method: Using the linear quadratic model, the relative effectiveness (RE) of a 20 Gy boost was calculated for tissue with an alpha/beta ratio rangin g from 2 to 10 Gy and a half-time of sublethal damage repair between 0 .1 and 3 h, The boost dose was considered to be delivered either in a number of pulses varying from 2 to 25, or continuously at a dose rate of 0.50, 0.80, or 1.20 Gy/h. Results: The RE of 20 Gy was found to be identical for PB in 25 pulses of 0.80 Gy each h and CLDR delivered at 0.80 Gy/h for any alpha/beta value and for a repair half-time > 0.75 h , When normal tissue repair half-times are assumed to be longer than t umor repair half-times, normal tissue sparing can be obtained, within the restriction of a fixed overall treatment time, with higher dose pe r pulse and longer period time (time elapsed between start of pulse n and start of pulse n + 1), An optimum relative normal tissue sparing l arger than 10% was found with 4 pulses of 5 Gy every 8 h, Hence, a the rapeutic gain might be obtained when changing from CLDR to PB by adjus ting the physical dose in such a way that the biological dose on the t umor is maintained. The normal tissue-sparing phenomenon can be explai ned by an increase in RE with longer period time for tissue with high alpha/beta ratio and fast or intermediate repair half-time, and the RE for tissue with low alpha/beta ratio and long repair half-time remain s almost constant. Conclusion: Within the benchmark of the LQ model, a dvantage in normal tissue-sparing is expected when matching the pulse frequency to the repair kinetics of the normal tissue exposed. A perio d time longer than 1 h may lead to a reduction of late normal tissue c omplications, This theoretical advantage emphasizes the need for bette r knowledge of human tissue-repair kinetics. (C) 1998 Elsevier Science Inc.