[Organizing pneumonia].

Organizing pneumonia is first of all a pathologic entity observed in different situations of pulmonary repair after aggression. It is characterized by the presence in the distal airways (alveoli, alveolar ducts, and respiratory bronchioli) of fibroblastic buds in a loose extracellular matrix. The same terminology also describes a clinical entity characterized by the appearance of patches of multiple alveolar opacities, especially suggestive when they are migratory. This entity is characterized by strong sensitivity to corticosteroids and the high frequency of disease relapses, which generally occur when the corticosteroid dose is reduced or in the weeks after treatment stops. Diagnostic certainty requires histologic proof, which is rarely obtained when the radiographic and clinical picture is typical. When it occurs without an identified cause, this entity is now known as cryptogenic organizing pneumonia (it was previously called bronchiolitis obliterans with organizing pneumonia, or BOOP). The same entity can be observed in other defined contexts (connective tissue disease, for example), or with identified causes (e.g., radiation therapy for breast cancer, drugs, or infections).