[Cryptogenic organizing pneumonia versus secondary organizing pneumonia]

Organizing pneumonia (OP) describes a histological pattern of acute or subacute lung damage. Clinically, patients present with cough, fever, and dyspnea. A distinction is made between idiopathic or cryptogenic organizing pneumonia (COP) and secondary organizing pneumonia (OP). In COP, neither clinical/radiological nor histological causes can be determined. It is classified as an interstitial idiopathic pneumonia (IIP) according to the criteria of the American Thoracic Society (ATS) and the European Respiratory Society (ERS). Secondary organizing pneumonia has a known triggering mechanism, such as infectious agents, certain medications, or concomitant symptoms of other primary pulmonary diseases and diseases of other organ systems. Common to both forms is the histological picture of intra-alveolar mesenchymal buds. These are myofibroblast proliferates that branch out along the alveolar spaces. They are usually accompanied by a moderate interstitial and alveolar, chronic, and macrophage-rich inflammatory cell infiltrate. The most important differential diagnosis is common interstitial pneumonia (UIP). It also shows fibroblast proliferates, which are, however, located in the interstitium. The correct classification of an IIP as a COP by means of clinical, radiological, and histological findings is essential, since the COP, in contrast to the UIP, responds very well to corticosteroids and therefore has an excellent prognosis compared to the UIP. The course of secondary organizing pneumonia depends on the respective underlying disease. Here it is important for the pathologist to correctly identify potential accompanying histological characteristics in order to be able to provide clues to a possible cause of OP.