Definition
• Lung damage caused by exposure to ionizing radiation; can be acute or chronic
Pathogenesis
• Injury induced by both the direct cytotoxic effect of ionizing radiation and indirectly due to inflammation and oxidant injury
Clinical features
Epidemiology
• Occurs in 5% to 15% of patients that receive radiation therapy
• Increased risk with concomitant use of chemotherapy
Presentation
• Dyspnea, coughing, and fever
• Onset between 2 to 3 months after therapy (ranging from 2 weeks to 6 months) for acute radiation pneumonitis
• Chronic radiation pneumonitis occurs 6 months after exposure
Prognosis and treatment
• Mild disease may resolve spontaneously
• Treatment with corticosteroids for severe disease may resolve symptoms within weeks
• Some patients develop fibrosis, which is the main determinant of persistent respiratory compromise
• Death due to radiation-induced pulmonary disease is rare
Pathology
Histology
• Microscopic findings may include diffuse alveolar damage (DAD), acute interstitial pneumonia, and interstitial lymphocytic infiltration in the early phase
• There may be variable interstitial, alveolar, and replacement fibrosis in the late phase
• Vascular intimal fibrosis with foamy macrophages can be seen
• Cells may display nuclear enlargement, hyperchromasia, and nuclear irregularity
Main differential diagnoses
• DAD; presence of hyaline membranes without atypia of cells
• Acute interstitial pneumonia
Fig 1 Radiation pneumonitis. Radiographic PA view (A) and CT scan images (B) of a patient with radiation pneumonitis.
(Courtesy of Dr. Christopher M. Strauss, Department of Radiology, University of Chicago, Ill.)
Fig 2 Radiation pneumonitis. Low power shows interstitial and alveolar fibrosis also involving a small bronchiole, which shows squamous metaplasia.
Fig 3 Radiation pneumonitis. Medium power highlights partial fibrosis of the bronchiole (bronchiolitis obliterans) and interstitial fibrosis.
Fig 4 Radiation pneumonitis. High power demonstrates single cell atypia. Both pneumocytes and fibroblasts are enlarged and have foamy cytoplasm.
Fig 5 Radiation pneumonitis. Some cells are markedly atypical with enlargement of both cytoplasm and nucleus.
Fig 6 Radiation pneumonitis. Lung architecture is distorted by fibrosis.
Fig 7 Radiation pneumonitis. Reactive fibroblasts are also atypical. Compare enlarged nuclei with those of normal inflammatory cells.