Inflammatory myofibroblastic tumor (IMT) is a neoplasm characterized by a proliferation of fibroblasts and myofibroblasts mixed with varying numbers of plasma cells, lymphocytes, macrophages, and other inflammatory cells on histopathologic examination.¹ IMT has formely been known under various other names including plasma cell granuloma, inflammatory pseudotumor (IPT), and xanthomatous pseudotumor.¹ IPT was regarded as an entirely reactive lesion until recently, when it became apparent that some cases represented a true myofibroblastic neoplasm evidenced by a clonal gene rearrangement involving chromosome 2p23, which encodes the ALK gene.² The term IMT is now preferred to define a true neoplasm, while the term IPT or plasma cell granuloma is used for nonneoplastic fibroinflammatory localized processes, which are not IMT or other specific entities such as IgG4-related disease (see Chapter 51). Under the rubric of IPT, various nonneoplastic processes were included in the past, such as pulmonary nodular lymphoid hyperplasia (also known as pseudolymphoma), postinfectious fibroinflammatory lesions, nodular presentation of organizing pneumonia, and other nonspecific fibroinflammatory
processes. The distinction of IMT from these other conditions is very important, and the term IPT should no longer be used to encompass IMT, which is a true neoplasm with somewhat unpredictable biologic behavior; IMTs, especially those in the extrapulmonary sites, have the potential for locally aggressive growth, recurrence, and even distant metastases in rare cases.

The lung is a relatively common site for IMT, along with abdominal cavity and retroperitoneum.^3,4,5,6 Pulmonary IMTs occur most commonly in children and young adults,⁵ but the observed age range is quite broad.^3,4 A subset of patients (up to 20%) with IMT will present with a systemic inflammatory syndrome, which may include fever, weight loss, malaise, anemia, thrombocytosis, leukocytosis, elevated sedimentation rate, and hypergammaglobulinemia.^3,5 Patients with pulmonary IMT may also present with localizing symptoms, especially if the tumor is endobronchial, which include cough, dyspnea, and hemoptysis.^4,5

Agrons et al. described the radiographic features in 61 cases of pulmonary IPT, which likely includes both IMT and nonneoplastic conditions.⁷ Fifty-two of their 61 patients (85%) had a solitary peripheral nodule or mass, and 11 (18%) patients had extraparenchymal involvement including hilar, mediastinal, and airway invasion. On computed tomography (CT) scan, 12 lesions had heterogeneous attenuation while 5 lesions were homogenous.⁷ FDG uptake in IMTs varies from low to high, which may be due to variability in tumor cellularity, biologic behavior of the tumor cells, the composition and proportion of inflammatory cells, and the extent of activation of the inflammatory cells, according to a retrospective study using 18F-FDG-PET/CT scan.⁸

IMTs are typically lobulated and circumscribed fleshy masses.³ They vary widely in size (1 to 15 cm) and may be endobronchial or parenchymal.^4,11 Most tumors are solitary, but multiple masses may occur.^5,11 Involvement of the chest wall or mediastinum is rare.^4,11