Radiation injury in mouse lung: dependence on oxygen levels in the inspired gas

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.