The optimum pressure of oxygen for radiotherapy of a mouse tumour

Our previous studies have shown that there is more regrowth delay in mammary tumours irradiated in C3H mice after 25 Gy when breathing normobaric oxygen than in those breathing air, as might be expected. However, in both cases this radiation response was reduced in anaesthetized animals in comparison with unanaesthetized control mice, when a time interval of only 10 min was allowed after anaesthesia. After 25 min, however, the response in air returned to the control level and the oxygen group now showed significantly more radiosensitization. We have now found that when tumour-bearing mice were exposed to different pressures of oxygen for that 25-min period after the induction of anaesthesia, before the tumours were treated with 25 Gy, there was even more regrowth delay with 2 atm pressure than 1, but that there was no further advantage from using 3 atm pressure of oxygen. Our data suggest that 2 atm may be the optimal pressure to use in anaesthetized mice and there is even a small benefit from using this pressure in unanaesthetized animals for a transplanted C3H mammary tumour.