Organizing pneumonia manifests as a histologic pattern characterized by loose plugs of connective tissue in the airspaces and distal airways. Interstitial inflammation and fibrosis are minimal or absent. Cryptogenic organizing pneumonia, or COP, is a distinctive clinical disorder among the idiopathic interstitial pneumonias, but the histologic pattern of organizing pneumonia is encountered in many different situations, including pulmonary infection, hypersensitivity pneumonitis, and collagen vascular diseases. Airspace consolidation is the cardinal feature of organizing pneumonia on chest radiographs and CT scans. In COP, the distribution is typically subpleural and basal and sometimes bronchocentric. Other manifestations of organizing pneumonia include groundglass opacity, tree-in-bud pattern, and nodular opacities. [from
HPO]