Inflammatory myofibroblastic tumor (IMT) of the lung (also known as plasma cell granuloma, inflammatory pseudotumor, fibrous histiocytoma, fibroxanthoma, and xanthogranuloma) includes a spectrum of pulmonary lesions. Such lesions most commonly present as solitary pulmonary nodules, but can also be locally invasive [1-4]. It is currently unclear whether these lesions represent a primary inflammatory process versus a low-grade malignancy with a prominent inflammatory response.
Similar lesions, referred to variably in the literature as plasma cell granulomas or inflammatory pseudotumors, can also develop in the orbit, skull base, thyroid, liver, spine, spleen, lymph nodes, and other tissues [1-3,5-9]. (See "Evaluation of peripheral lymphadenopathy in adults".)
IMTs can occur in any age group, but over half of the patients are less than 40 years of age . Although IMTs are rare, comprising less than 1 percent of all surgically resected lung lesions, they do represent one of the most common primary lung tumors in the pediatric age group [4,11-16]. Both sexes and all ethnic groups appear to be equally affected.
There is significant controversy and confusion regarding the pathogenesis and histogenesis of these uncommon tumors or tumor-like masses (picture 1). Much of the confusion has been caused by the varying degrees of inflammatory cell infiltration noted on pathologic examination and the observation that the disease process, although usually following a benign course, is sometimes invasive. As a result, a variety of terms have been used to describe lesions falling under the category of IMT.
The early terminology "pulmonary plasma cell/histiocytoma complex" emphasized the histologic heterogeneity and a belief in the benign nature of the lesions . Subsequently, plasma cell granuloma was the common terminology for lung lesions, acknowledging the circumscribed appearance, presence of plasma cells and histiocytes, and generally benign course. The term inflammatory pseudotumor was typically used to describe extrapulmonary lesions with similar pathology [18-20]. More malignant variants were felt to be analogous to undifferentiated “pleomorphic” sarcomas (previously termed malignant fibrous histiocytoma) arising in soft tissues. (See "Clinical presentation, histopathology, diagnostic evaluation, and staging of soft tissue sarcoma", section on 'Histopathology'.)