Injurious effects of radiotherapy on the lungs. There are three phases in the reaction of the lungs to radiation injury: an acute phase occurring 1 to 2 months after exposure, a subacute phase 2 to 9 months after exposure, and a chronic or fibrotic phase more than 9 months after exposure. In the acute phase there is vascular damage, congestion, edema, and mononuclear cell infiltration; in the subacute phase the alveolar walls are infiltrated with mononuclear inflammatory cells and fibroblasts; in the chronic phase, alveolar fibrosis and capillary sclerosis take place. (Cecil Textbook of Medicine, 19th ed, p2343).