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

Abstract

The effectiveness of localized X-irradiation on the transplantable KHT sarcoma was investigated while the host mice breathed gas mixtures ranging from 5 per cent O₂ : 95 per cent N₂ to 100 per cent 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 ≃ 50 per cent O₂). This relationship holds for the two doses and the three anaesthetics studied, and indicates the importance of investigating the PaO₂ levels of patients undergoing radiotherapy treatment.