Contributors: Diva R. Salomão and R. Jean Campbell
Malignant fibrous histiocytoma is a malignant mesencymal neoplasm of soft tissue characterized by a hypercellularity of pleomorphic spindle shaped cells with nuclear irregularity, hyperchromatism, and numerous mitotic figures. Malignant fibrous histiocytoma infiltrates the surrounding tissues and this results in incomplete surgical excision so that repeated local recurrences are common. The tumor causes death in some patients because of a direct invasion of vital structures or distant blood borne metastasis. The 10 year survival rate is ~ 25%. Malignant fibrous histiocytoma needs to be differentiated from a locally aggressive fibrous histiocytoma that lacks a metastatic potential. A locally aggressive fibrous histiocytoma manifests a distinct infiltrative attribue, but lacks cytological features of malignancy. It usually presents in adult life, but have been observed in infancy. There is no gender predilection. Malignant fibrous histiocytoma may arise from abenign fibrous histiocytoma. Proptosis and diplopia are caused by an infiltration of the extraocular muscles.The tumor is believed to arise from facultative phagocytes rather than primary histiocytes. A miagnant fibrous histocytoma lacks a true capsule and has histologic features of a fibrous hisioctoma, such as spindle-shaped cells that form a characteristic storiform (matted) or cartwheel pattern. Extracellular connective tissue is variable. Cells with a histiocytic morphology complemented by appropriate enzyme properties are an inconstant feature. Scattered multinucleated giant cells are sometimes present. Mitotic activity is prominent. The tumor cells have interdigitating processes and form rudimentary desmosomal junctions. This morphology is typical of fibroblasts rather than of macrophages. HRAS mutations have been detected in numerous malignant fibrous histiocytomas.