Inflammatory myofibroblastic tumor, inflammatory fibrosarcoma, and related lesions: An historical review with differential diagnostic considerations

The concept of the inflammatory myofibroblastic tumor (IMT) has evolved from an already perplexing pathological process, the inflammatory pseudotumor, which was initially recognized in the lung and regarded as a pseudoneoplasm, although its histological features resembled a spindle cell sarcoma. Despite the pathological findings and their apparent prognostic implications, most affected individuals regardless of the primary site have had favorable clinical outcomes. The designation of inflammatory pseudotumor came to be widely accepted, although these lesions were clearly tumors or masses that may or may not have been pseudoneoplasms. An aberrant or exaggerated response to tissue injury without an established cause has generally been favored as the pathogenesis of the inflammatory pseudotumor or IMT. Once the myofibroblast was identified and its function in tissue repair was established, this cell type was found in a variety of soft tissue lesions from nodular fasciitis to malignant fibrous histiocytoma. The myofibroblast was eventually recognized as the principal cell type in the inflammatory pseudotumor, which provided the opportunity to redesignate this tumor as IMT. Some of the clinical and pathological aspects of the IMT began to suggest the possibility that these lesions are more similar to neoplasms than a postinflammatory process. Another step in the evolution of the inflammatory pseudotumor and IMT occurred with the report of a mesenteric or retroperitoneal tumor with similar pathological features to the latter tumors but with more aggressive behavior to warrant an interpretation of malignancy as an inflammatory fibrosarcoma. The IMT and inflammatory fibrosarcoma appear to have many overlapping clinical and pathological features. These tumors are histogenetically related, and if they are separate entities, they are differentiated more by degrees than absolutes. The therapeutic approach to these tumors should relay primarily on surgical resection. Studies in the future may possibly resolve the question whether the IMT and inflammatory fibrosarcoma are synonomous or closely related entities.