Many issues regarding inflammatory pseudotumor and inflammatory myofibroblastic tumor (IMT) are unresolved, including the usage of these terms. Different investigators have different concepts of these entities, and it appears that various lesions have been included under this diagnosis. Traditionally, in the lung, inflammatory pseudotumors were considered nonneoplastic reactive lesions. In the past, synonyms for inflammatory pseudotumor included plasma cell granuloma. However, even in years past, there were cases of so-called inflammatory pseudotumors of the lung that were multiple, recurrent, or locally invasive. In retrospect, many of the inflammatory pseudotumors with these more aggressive features were probably neoplasms. IMTs are distinctive lesions composed of myofibroblastic spindle cells accompanied by inflammatory cells; they occur primarily in soft tissues but occasionally in the lungs. IMTs have also been called inflammatory pseudotumors. There is disagreement among investigators as to whether or not any or all IMTs are neoplastic. Further confusion is added by inclusion of IMTs with frank clinical and pathologic malignant features as inflammatory fibrosarcomas. Because of the traditional classification of inflammatory pseudotumors as non-neoplastic, we discuss these entities in this section, fully recognizing that some of these are probably true neoplasms.

The following discussion concerns both the traditional non-neoplastic lesions in the lung and IMTs in the lung. IMTs are histologically distinctive and yet the pathologist is more likely to encounter wedge resections of traditional non-neoplastic inflammatory pseudotumors. It is recognized that some lesions overlap histologically or may not be specifically classifiable.

The term inflammatory pseudotumor of the lung was first introduced in the 1950s to characterize pulmonary masses with propensity to mimic a malignant process clinically and radiologically. Similar to focal organizing pneumonias, these lesions present as radiologic masses suspected of being malignancies and are found to be purely inflammatory on histopathologic examination. Based on the predominant histopathologic features, these lesions have been divided into three types: (i) organizing pneumonia pattern, (ii) fibrous histiocytic pattern (most common), and (iii) lymphohistiocytic pattern (least common). The etiology of inflammatory pseudotumors is unknown, but an antecedent pulmonary insult, usually infection, is found in about 20% of cases.

IMT represents a subgroup of the broad category of “inflammatory pseudotumors.” Some authors believe IMT is a reactive inflammatory condition, whereas others believe it represents a low-grade mesenchymal malignancy. It presents as a well-defined mass on chest radiographs in the majority of cases. Grossly, IMTs consist of multilobular or bosselated tumors with a rubbery surface and a yellowish-white or gray appearance on sectioning. Pulmonary IMTs have been associated with viral infections including HHV-8. In most cases, surgical excision is curative, with a minority of cases exhibiting recurrence, extrapulmonary invasion, and metastases. Some authors continue to use the terms inflammatory pseudotumor and IMT interchangeably. Other investigators suggest that IMTs that exhibit frank malignant behavior and/or histopathologic features of malignancy are best designated as inflammatory fibrosarcomas.