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-d ose-rate brachytherapy. However, apart from data obtained from modeling, on ly a few irt vitro results are available for comparing the biological effec tiveness of both modalities. Cells of two human cell lines with survival fr actions 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 use d. The cells were irradiated with a continuous low dose rate (80 cGy per ho ur) 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 tre atment time constant. Survival after low-dose-rate irradiation was indistin guishable 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 durin g the pulses was increased, survival decreased even further. This effect wa s most pronounced for the radiosensitive HX142 cells, In clinical pulsed-do se-rate brachytherapy, iridium sources move stepwise through the implant an d deliver pulses at a high dose rate locally. These high-dose-rate pulses p roduce greater biological effectiveness compared to continuous low dose rat e; this should be taken into account. (C) 1999 by Radiation Research Societ y.