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Medical Definition of Radiation pneumonitis
1. 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, oedema, 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) (12 Dec 1998)