Radiation Injury in Mouse Lung: Dependence on Oxygen Levels in the Inspired Gas

Summary

The relationship between radiosensitivity and the partial pressure of oxygen (PO₂) in the inspired gas has been established for radiation pneumonitis as a measure of lung damage following irradiation of the mouse thorax. The radiosensitivity at low PO₂ (0–1 per cent) fitted the linear transformation of the Alper, Howard-Flanders relationship giving a K value for lung tissue of 1·35 per cent oxygen with an oxygen enhancement ratio, m, of 2·13. The radiosensitivity at higher PO₂ (5–21 per cent) did not fit the Alper, Howard-Flanders relationship probably because the PO₂ of the inspired gas was greater than the PO₂ in the alveolus. At the low PO₂ levels in the inspired gas, back diffusion of oxygen from blood into the alveolus may lead to errors in the estimated value of K. If the low value of m is due to this ‘contaminating’ oxygen from blood then by taking a higher value for m, the amount of contaminating oxygen can be calculated (0·23 per cent) and a ‘true’ value for K (1·1 per cent) determined. Other uncertainties in this estimate of K due to the radiolytic consumption of oxygen and possible inadequacies in equilibration are discussed. Allowing for the uncertainties, it is concluded that the K value for lung damage lies towards the upper end of the range of K values measured for cells in vitro.