Cardiovascular sequelae of therapeutic thoracic radiation. Mediastinal radiation damages endothelial cells, with resulting loss of capillaries and ischemia at the level of the microcirculation. These changes lead to increases in collagen and proliferation of fibrous tissue throughout the heart. Cardiac dysfunction following radiotherapy is surprisingly common and may be due to pericardial, myocardial, valvular, conduction system, or coronary artery disease. Greater awareness of cardiotoxicity has prompted changes in radiation techniques that appear to reduce clinical cardiovascular complications. 