Pathophysiology
Clinical meaning
Radiation pneumonitis develops 1-6 months after thoracic irradiation when ionizing radiation damages type I and type II pneumocytes and pulmonary capillary endothelial cells. Free radical-mediated DNA damage triggers apoptosis of alveolar lining cells, leading to surfactant depletion, increased capillary permeability, and alveolar edema. The inflammatory response involves macrophage and lymphocyte infiltration with release of pro-fibrotic cytokines (TGF-beta, TNF-alpha). Without treatment, the acute inflammatory phase progresses to irreversible radiation fibrosis within 6-24 months.