The sequence of histological changes in mouse lungs after single doses of X-rays

Histological changes in mouse lungs have been quantified up to one year after irradiation of both lungs with a range of single X-ray doses. Three lesions characteristic of the acute, intermediate, or late phase of lung response were scored separately according to the area of the miscroscopic section involved. An “average score” was obtained for each lesion and plotted against time. Clearly separated peak response times were found for the acute phase (18 weeks) and intermediate phase (36 weeks). The peak response times of these 2 phases were independent of dose. Fibrosis was just detectable histologically at 36 weeks after 14 Gy and 13 Gy. The severity of each phase was dose related. The mice that died did so mostly between 14 and 24 weeks (98 and 168 days), the normal time over which death occurs in lung LD₅₀ studies. The histological data indicated that the lung LD₅₀ assay is related to acute radiation pneumonitis rather than to late fibrosis.