Inflammatory myofibroblastic tumor (IMT) is a rare neoplasm of the mesodermal cells that form the connective tissues which support virtually all of the organs and tissues of the body. IMT was formerly termed inflammatory pseudotumor. Currently, however, inflammatory pseudotumor designates a large and heterogeneous group of soft tissue tumors that includes inflammatory myofibroblastic tumor, plasma cell granuloma, xanthomatous pseudotumor, solitary mast cell granuloma, inflammatory fibrosarcoma, pseudosarcomatous myofibroblastic proliferation, myofibroblastoma, inflammatory myofibrohistiocytic proliferation, and other tumors that develop from connective tissue cells. Inflammatory pseudotumour is a generic term applied to various neoplastic and non-neoplastic tissue lesions which share a common microscopic appearance consisting of spindle cells and a prominent presence of the white blood cells that populate chronic or, less commonly, acute inflamed tissues.Inflammatory myofibroblastic tumor was initially regarded as a benign tumor that most often developed in the lung and less commonly in almost any organ system or tissue. Over time, however, IMT cases occurred in which the tumor spread into local tissues, metastasized to distal tissues, recurred after treatment, or consisted of neoplastic cells that had pro-malignant chromosome abnormalities. Consequently, the World Health Organization, 2013, and current literature commonly describe inflammatory myofibroblastic tumor as a neoplasm with intermediate malignant potential or a rarely metastasizing neoplasm. In 2020, the World Health Organization reclassified IMT as a specific tumor form in the category of intermediate (rarely metastasizing) fibroblastic and myofibroblastic tumors.
However, a more recent retrospective study of 92 patients accumulated by the Surveillance, Epidemiology, and End Results (SEER) program of the National Cancer Institute found the mean age of disease onset was 47.4 years with peak occurrences at 0 to 4, 36 to 40, and >50 years old; middle-aged individuals (41 to 64 years) represented 1/3 of all cases. In this study, the commonest sites of tumor occurrence were the lower limb and hip (22% of cases), upper limb and shoulder (12% of cases), and head, face, and neck (9% of cases). Another recent study of 25 patients found the commonest sites of IMT were the abdomen (40% of cases) and lung/thoracic wall (32% of cases). Individual IMT cases are also reported to present in the urinary bladder, anal canal, and parameningeal spaces (i.e. sites adjacent to the meninges such as the nasopharynx, middle ear, paranasal sinuses, infratemporal fossa and pterygopalatine fossa). Apparently, the age and organ/tissue distribution of IMT various with the patient population examined: in general it can present in individuals of almost any age and in almost any organ or tissue site. IMT most commonly presents as a tumor localized to a single site but may be associated with distal metastases in up to 5% of all cases or up to 10% of cases in which the tumor cells express an ALK fusion protein. The tumors range in size from 1–25 cm (average 6.5 cm) with two-thirds being 1.5–6.5 cm.