Inflammatory pseudotumors of the lung: progression from organizing pneumonia to fibrous histiocytoma or to plasma cell granuloma in 32 cases.

Clinicopathologic findings in 32 cases of inflammatory pseudotumor of the lung are described. Depending on the major histopathologic features, the cases are divided into three groups: Organizing pneumonia type (44%), fibrous histiocytoma type (44%), and lymphoplasmacytic type (12%). Organizing pneumonia type has intraalveolar lymphohistiocytic inflammation which converts to intraalveolar fibrosis peripherally and interstitial fibrosis centrally because of a proliferation of fibroblasts. Fibrous histiocytoma type has a predominant proliferation of spindle cells and histiocytes in storiform pattern, with loss of alveolar architecture. Lymphoplasmacytic type has a predominance of lymphocytes and plasma cells with little fibrosis. There is considerable histologic overlap among the three types. All cases have regions of organizing pneumonia. Most or all cases of inflammatory pseudotumor are believed to originate as organizing pneumonia.