Fibrous histiocytoma exists as a benign or malignant neoplasm.
Malignant fibrous histiocytoma (MFH) occurs at any age. Peak incidence of the tumor is in the seventh decade of life.
MFH is slightly more common in males.
MFH occurs most commonly in the soft tissues more frequently (50%) in the lower extremities, less frequently in the upper extremities (20%), and retroperitoneum (20%). MFH also occurs in the bone usually around the knee, and less frequently in the lower or upper extremities and pelvis.
Grossly, the soft tissue tumors appear as gray-white, multi-lobulated mass which is not encapsulated and has an infiltrative border. The tumor of bone may erode through the cortex and penetrate the soft tissues. The tumor has a fleshy, soft, translucent cut surface and may show areas of hemorrhage and necrosis.
Microscopically, five different variants of MFH is recognized. They include the storiform-pleomorphic, myxoid, giant cell, inflammatory and angiomatoid variants. The storiform-pleomorphic and the mxyoid types are more common. The storiform-pleomorphic variant consists of polygonal histiocytes, fibroblasts/myofiborblasts and mesenchymal cells. The histiocytes have oval nuclei and cytoplasmic extensions. The fibroblastic/myofibroblastic cells are spindle shaped and exhibit a storiform (cartwheel) pattern, which is commonly found around slit-like vessels. The mesenchymal cells are oval to polygonal and have small nuclei. The tumor cells may exhibit pleomorphism and mitotic activity. In addition to these cells, other cell types including, xanthoma cells, benign giant cells, lymphocytes, plasma cells and giant tumor cells exhibiting hyperchromatic nuclei may be seen. The myxoid variant exhibits areas of myexoid stroma rich in acid mucopolysaccharides. Cellular areas of the tumor are similar to that seen in the storiform variant and consists of mesenchymal cells. These cells may show cytoplasmic vacuoles that contain the acid mucopolysaccharides. The differential diagnosis of this variant includes, myxoid liposarcoma, nodular fasciitis, and myxoma.