Radiation Induced Lung Damage: Mechanisms and Clinical Implications

Radiation therapy is one of most important therapeutic modalities for thoracic malignancies. However, radiation-induced lung damage, such as radiation pneumonitis or fibrosis, is a main dose-limiting factor when irradiating the thorax. The radiation over threshold dose results in damage to pneumocytes and endothelial cells and the inflammatory changes following the damage lead to necrosis of damaged tissue, which are then replaced by fibrotic tissue. There is diffuse lung damage and edema on histopathologic inspection; however, the tissue damage and edema is not specific for radiation injury and we are far from a reliable pathogenic model. Many parameters have been evaluated for predicting radiation pneumonitis and the most consistent predictor is cumulative radiation dose to normal lung tissue. The combination of chemotherapy probably increases the incidence and severity of radiation pneumonitis; however, this is not clear. Efforts to reduce the radiation dose to normal lung tissue using new radiotherapy techniques can reduce the incidence and severity of radiationinduced lung damage. Many biological agents have been tried to prevent and treat radiation pneumonitis; however, more data is needed.