The relationship between mouse arterial partial pressure of oxygen (PaO2) and the effectiveness of localized tumour irradiation.

The effectiveness of localized X-irradiation on the transplantable KHT sarcoma was investigated while the host mice breathed gas mixtures ranging from 5% O2 : 95% N2 to 100% O2. Subcutaneous tumours in the flank of C3H mice anaesthetized with Nembutal (sodium pentobarbitol), Valium (diazepam), or urethane, were irradiated with doses of 1,500 or 2,500 rads and the delay in days for the tumours to grow from 4 to 10 mm in diameter was determined. Measurements of the arterial partial pressure of oxygen (PaO2) were made under conditions identical to the irradiation experiments. For PaO2 values less than 100 mm Hg (animals breathing air), growth delay decreased linearly, while for PaO2 values greater than 100 mm Hg, growth delay increased and reached a plateau at a PaO2 between 200 and 300 mm Hg, (animals breathing congruent to 50% O2). This relationship holds for the two doses and the three anaesthetics studied, and indicates the importance of investigating the Pa02 levels of patients undergoing radiotherapy treatment.