Organizing pneumonia (OP) is a pathological pattern defined by the characteristic presence of buds of granulation tissue within the lumen of distal pulmonary airspaces consisting of fibroblasts and myofibroblasts intermixed with loose connective matrix. This pattern is the hallmark of a clinical pathological entity, namely cryptogenic organizing pneumonia (COP) when no cause or etiologic context is found. The process of intraalveolar organization results from a sequence of alveolar injury, alveolar deposition of fibrin, and colonization of fibrin with proliferating fibroblasts. A tremendous challenge for research is represented by the analysis of features that differentiate the reversible process of OP from that of fibroblastic foci driving irreversible fibrosis in usual interstitial pneumonia because they may determine the different outcomes of COP and idiopathic pulmonary fibrosis (IPF), respectively. Three main imaging patterns of COP have been described: (1) multiple patchy alveolar opacities (typical pattern), (2) solitary focal nodule or mass (focal pattern), and (3) diffuse infiltrative opacities, although several other uncommon patterns have been reported, especially the reversed halo sign (atoll sign). Definitive diagnosis is based on (1) a suggestive clinical radiological presentation, (2) the demonstration of the characteristic pathological pattern at lung histopathology, and (3) exclusion of possible causes. Transbronchial biopsies or a transthoracic biopsy may also contribute to the pathological diagnosis. Rapid clinical and imaging improvement is obtained with corticosteroid therapy. Because of the risk of misdiagnosing alternative conditions that may mimic OP, only typical cases may be managed without histopathological confirmation, and patients should be followed with particular attention paid to any clue of alternate diagnosis, especially in case of incomplete response to treatment. Patients and clinicians must be aware of frequent relapses after stopping corticosteroid treatment.
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