A clinicopathologic comparison of malignant fibrous histiocytoma and liposarcoma

The diagnosis of MFH depends on the demonstration of histiocytic and fibroblastic functions. MFHs may phagocytose fat; therefore, lipid stains are useless. By electron microscopy, cytoplasmic lipid is membrane-bound. Immunohistochemical staining for vimentin and histiocytic markers may be helpful. Liposarcoma is diagnosed only when there is convincing evidence of synthesis and storage of fat by the tumor cells. By electron microscopy, cytoplasmic lipid is nonmembrane-bound. Both MFH and liposarcoma have subtypes. In MFH, the pleomorphic forms are the most common. Myxoid MFH is less common; all other types are rare. In liposarcoma, the myxoid types are by far the most common. The myxoid types of both MFH and liposarcoma may contain other elements that vary in degree and geographic distribution and that can raise the histologic grade. About 50% of liposarcomas are low-grade tumors; these are almost always purely myxoid. Low-grade myxoid liposarcoma has a much better prognosis than other types. Myxoid liposarcoma has a better prognosis than myxoid MFH. The peak incidence of MFH is in the seventh decade of life whereas that of liposarcoma is in the fifth decade. A substantial number (roughly 25% to 30%) of MFHs occur in the subcutaneous tissue. Clinically they are almost invariably mistaken for ganglion cysts. Liposarcoma, however, is likely to occur in or below the buttocks. Most are in the anterior thigh. Subcutaneous liposarcoma is extremely rare. A tumor in this area is likely to be either a more malignant myxoid MFH or one of the benign atypical lipomatous tumors. In both MFH and liposarcoma, the development of distant lesions is related to the tumor's histologic grade and size and to local recurrence. Favored metastatic sites of MFH are lung and lymph nodes. Favored sites of distant lesions in the myxoid/round cell types of liposarcoma are intra-abdominal, retroperitoneal, other soft-tissue areas (especially in the neck), and bone. Lymph node involvement is very rare. Because myxoid/round cell liposarcomas have a marked propensity to involve intra-abdominal sites, abdominal computed tomography and bone scan are recommended in the initial evaluation and follow-up of high-risk patients (those with high-grade tumors larger than 15 cm and those with local recurrence of intermediate- or high-grade tumors of any size).