Myxoid and round cell liposarcoma: A spectrum of myxoid adipocytic neoplasia

Myxoid and round cell liposarcoma accounts for about 30% to 35% of all lipo sarcomas and, even if still classified by the World Health Organization (WH O) as 2 distinct subtypes, share both clinical and morphologic features. Le sions combining both patterns are frequent and wide agreement exists in con sidering round cell liposarcoma as the high grade counterpart of myxoid lip osarcoma. Furthermore, myxoid and round cell liposarcoma share the same cha racteristic chromosome change represented most frequently by a reciprocal t ranslocation t(12;16)(q13:p11) that fuses the CHOP gene with the TLS gene. Clinically, myxoid and round cell liposarcoma tend to occur in the limbs wi th a peak incidence ranging between the third and the fifth decade and exhi bit overall a metastatic rate of approximately 30%. A peculiar tendency to metastasize to the soft tissue is observed that should not be interpreted a s multicentricity. Microscopically, purely myxoid liposarcoma is composed b y a hypocellular spindle cell proliferation set in a myxoid background and associated with a varying number of monovacuolated lipoblasts. The most hel pful morphologic clue is represented by the presence of a thin-walled capil lary network organized in a plexiform pattern. The most important morpholog ic variation observed in myxoid liposarcoma is represented by the occurrenc e of hypercellular areas that may exhibits an undifferentiated round cell m orphology. On the basis of the percentage of hypercellularity/round cell fo rmation, a myxoid/round cell liposarcoma (more than 25% hypercellular/round cell areas) and a round cell liposarcoma (more than 75% hypercellular/roun d cell areas) are somewhat arbitrarily recognized. Both the recognition and the quantification of hypercellular/round cell areas represents a crucial step in the evaluation of this liposarcoma subtype because hypercellularity appears to correlate with the clinical outcome. In consideration of the in trinsic difficulty in establishing accurately the percentage of high grade areas as well as of application of different cut off values, it appears saf er to consider any amount of hypercellularity as prognostically relevant. C areful as well as extensive sampling is mandatory to permit detection of th e smallest amount of hypercellularity. The differential diagnosis of myxoid liposarcoma includes benign lesions, such as myxoid spindle cell lipoma, i ntramuscular myxoma and lipoblastoma, and malignant ones such as low grade myxofibrosarcoma, and extraskeletal myxoid chondrosarcoma. In consideration of the great morphologic variability, the application of both immunohistoc hemistry and genetics has proved helpful in sorting out the more challengin g cases. Copyright (C) 2001 by W.B. Saunders Company.