Differential effects of dose rate and superfractionation on survival and cell cycle of V79 cells from spheroid and monolayer culture

Recent developments concerning brachytherapy suggest conditions for an equivalence between the common continuous low dose rate (CLDR) exposure and pulsed irradiation regimens (PDR), provided that total dose is administered in the same overall time. The respective theoretical considerations have been based solely on the phenomenon of sublethal damage recovery. The present study, therefore, aimed to assess a possible influence of growth state/cell cycle progression when CLDR and different super fractionation protocols are compared. The respective experiments were performed with V79 cells that can be grown as a rapidly proliferating monolayer culture or as small spheroids (without hypoxia) where most of the cells are out of cycle. Differential changes in cell cycle distribution occuring during the compared exposure schemes and their impact on cell survival were expected to be expressed most clearly with this model system because of the short G(1) phase. Cell irradiations were performed with brachytherapy sources either continuously (Cs-137) Or With high dose rate pulses (Ir-192) at different (1 h and 4 h) pulse repetitions whereby the overall dose rate was kept constant to approximately 1 Gy/h. Cell survival curves were generated by sampling cells at different exposure times or number of pulses, respectively. For spheroid cells an unequivocal decrease of effectivity was demonstrated with decreasing dose per pulse, and the dose effect relation obtained with hourly pulses of 1 Gy was indistinguishable from the CLDR response. For monolayer cells, on the contrary, the scheme of hourly pulses was significantly more effective than the CLDR irradiation. As measured by flow cytometry, this different behaviour could be attributed to the accumulation of cycling cells in the radiosensitive G(2)/M phase (G(2) block) during protracted exposure which was drastically more pronounced for the pulsed scheme compared to the CLDR condition. The observed principle phenomenon of a block to cell cycle progression from high dose rate pulses (at low overall dose rate) may be less expressed in (human) cells having a long G(1) period, but if applicable to a clinical situation, an increase of acute effectiveness of a superfractionated brachytherapy protocol has to be considered.