Cryptogenic organizing pneumonia (COP), formerly termed idiopathic bronchiolitis obliterans organizing pneumonia (idiopathic BOOP), consists of proliferation of granulation tissue within small airways, alveolar ducts, and alveoli. The clinical syndrome occurs most often in middle-aged to older adults and is often preceded by a flu-like illness. Persistent nonproductive cough and shortness of breath are the usual presenting symptoms. Most, but not all, patients respond rapidly to steroids, and in most cases prognosis is excellent.

COP is an idiopathic clinical syndrome, but similar lesions can be seen in various specific pulmonary injuries (see Chapter 62). These identifiable causes of organizing pneumonia must be excluded clinically and pathologically before a diagnosis of COP can be made. When an organizing pneumonia pattern is present, the pathologist should examine the tissue for viral inclusions, poorly formed granulomas or isolated giant cells (hypersensitivity pneumonitis), and foreign-body giant cells (aspiration).

At low power, COP consists of a patchy pattern of nodules of granulation tissue that typically center on a small airway surrounded by a normal or near normal lung. At higher power there are arborizing branches of granulation tissue (fibroblasts in an edematous or myxoid stroma) that fill the lumens of bronchioles, alveolar ducts, and adjacent alveoli. Rounded plugs of granulation tissue in the alveolar spaces are referred to as Masson bodies. The general architecture of the lung is preserved, with no significant interstitial fibrosis or honeycombing. There may be small foci of lymphocytes, plasma cells, and macrophages in some bronchioles, and interstitial inflammation is usually minimal to moderate. Transbronchial biopsies may fail to sample bronchioles and sample only alveoli with organizing pneumonia. Intra-alveolar collections of foamy macrophages may result from the bronchiolar obstruction.