Pulsed-dose-rate and low-dose-rate brachytherapy: Comparison of sparing effects in cells of a radiosensitive and a radioresistant cell line

Pulsed-dose-rate regimens are an attractive alternative to continuous low-dose-rate brachytherapy. However, apart from data obtained from modeling, only a few irt vitro results are available for comparing the biological effectiveness of both modalities. Cells of two human cell lines with survival fractions of 80% (RT112) and 10% (HX142) after a single dose of 2 Gy and with different halftimes for split-dose recovery and low-dose recovery were used. The cells were irradiated with a continuous low dose rate (80 cGy per hour) or with pulsed dose rate. Two different pulsed dose rates were tested: 4.25 Gy/h and 63 Gy/h, The effects of dose per pulse and the length of the interval between the pulses were investigated while keeping the overall treatment time constant. Survival after low-dose-rate irradiation was indistinguishable from that after pulses of 4.25 Gy/h in cells of both cell lines. Survival decreased with increasing dose per pulse, When the dose rate during the pulses was increased, survival decreased even further. This effect was most pronounced for the radiosensitive HX142 cells, In clinical pulsed-dose-rate brachytherapy, iridium sources move stepwise through the implant and deliver pulses at a high dose rate locally. These high-dose-rate pulses produce greater biological effectiveness compared to continuous low dose rate; this should be taken into account. (C) 1999 by Radiation Research Society.