In all events, IMT is a rare tumor with a reported incidence in 2009 of 150–200 cases/year in the United States.IMT lesions typically consist of, and are defined by, myofibrolastic spindle cells, i.e. specialized cells that are longer than wide, have a microscopic appearance that merges the appearances of fibroblasts and smooth muscle cells (see myofibroblast), occur in normal as well as tumor tissues, and in normal tissues are commonly designated  fibroblasts. However, the lesions in some IMF cases are dominated by sheets of epithelioid cells (which may have rounded shapes) with only a minor component of spindle cells. Tumors with these characteristics  are regarded as a subtype of IMT termed epithelioid inflammatory myofibroblastic sarcoma (EIMS).The tumors in IMT and EIMS consistently contain pro-inflammatory white blood cells and in most cases tumor cells that express highly abnormal oncogenic (cancer-causing) fusion proteins such as those that contain the active portion of anaplastic lymphoma kinase (ALK). It is not clear whether this inflammation, the genetic abnormalities, or both contribute to the development of IMT but drugs blocking the activities of the fusion proteins made by these genetic abnormalities may be useful in treating the disease. Signs and symptoms
IMT was regarded as a tumor that occurs in children or young adults and presented in the lung, mesentery, greater omentum or, less commonly, heart, liver, spleen, pancreas, colon, small intestine, spermatic cord, prostate, uterus, eye orbit, peripheral or central nervous system nerves, brain meninges, spinal cord, or other sites.
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.