Split-dose and low dose-rate recovery in four experimental tumour systems

The radiation dose-rate effect has been investigated in three murine tumour cell systems (MT carcinoma, Lewis lung tumour, B16 melanoma) and in the HX34 human melanoma xenograft taken directly from mice and irradiated in vitro. The four tumour types were remarkably similar in their radiation response characteristics, especially at low dose rate; the Lewis lung tumour tended to be the most radiosensitive at high dose rate. The data have been analysed using the Lethal-Potentially Lethal (LPL) model of Curtis and the Incomplete Repair (IR) model of Thames. The data are equally well fitted by both models. The most remarkable feature of these analyses is that both models lead to estimates for the half-time for recovery that are in the region of 0.1 h, considerably shorter than other published values. Split-dose experiments were also performed, taking care to keep the cells at 37 degrees C between exposures. In all cases the split-dose half times were longer than the values derived from dose-rate analysis and in the case of the Lewis lung and HX34 tumour lines the difference was by almost a factor of ten. The discrepancy between these estimates could be the result of biphasic cellular recovery.