Organizing pneumonia (formerly referred to as bronchiolitis obliterans organizing pneumonia [BOOP]) is seen secondary to a variety of lung injuries and as a component of several specific lung diseases. The same histologic pattern occurs as an idiopathic clinical syndrome called cryptogenic organizing pneumonia (COP; formerly referred to as idiopathic bronchiolitis obliterans organizing pneumonia), classified with the idiopathic interstitial pneumonias (see Chapter 94).

Organizing pneumonia may occur in viral or other infections, as a drug reaction, as a reaction to chemotherapy or radiation therapy, after inhalation of fumes or toxic compounds, as a postobstructive finding distal to an obstructed airway, as a result of aspiration, and in bone marrow transplant recipients. It is proposed that this pattern may represent acute lung transplant rejection in some lung transplant patients (see Chapter 109). This pattern can also be a component of specific lung diseases, including hypersensitivity pneumonitis, eosinophilic pneumonia, collagen vascular diseases involving the lungs, and Wegener granulomatosis.

Organizing pneumonia consists of plugs of granulation tissue (fibroblasts in an edematous or myxoid stroma) in the lumens of bronchioles, alveolar ducts, and adjacent alveoli. Rounded nodules of granulation tissue in alveolar spaces are called Masson bodies. There may be accompanying interstitial lymphocytes or other inflammation. Transbronchial biopsy may fail to sample bronchioles, and the only finding may be the granulation tissue in the alveoli. Intra-alveolar collections of foamy macrophages may result from the bronchiolar obstruction. There may be findings that suggest the etiology (e.g., viral inclusions in viral pneumonias, foreign-body giant-cells in aspiration, poorly formed granulomas, and multinucleated giant-cells in hypersensitivity pneumonitis). Histopathologic clues to the etiology of organizing pneumonia may not be present, and clinical correlation is often necessary to determine the underlying cause. If an identifiable etiology is excluded, then the diagnosis is cryptogenic organizing pneumonia (see Chapter 94). The organizing pneumonia may resolve with or without residual scarring.