10 June 2005 – Case of the Week #10
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thank Dr. David Lucas, University of Michigan Hospitals,
Case of the Week #10
A 30 year old man presented with a mass in the inner thigh. MRI disclosed an enhancing, heterogeneous mass in the thigh, abutting the femur (image #1). Following open biopsy, he was treated with adriamycin-based chemotherapy, and the tumor was resected. Grossly, it was largely gelatinous, but had areas that appeared fleshy (image #2). Eighteen months later he developed recurrent disease in the groin, and he ultimately died of disseminated disease.
Micro description: The tumor showed variation from field to field. Some of the tumor was relatively hypocellular with myxoid stroma, low grade nuclear cytology, plexiform small vascular pattern, and readily identified signet ring type lipoblasts (image #3). However, most of the tumor was more cellular, consisting of closely-spaced, overlapping, anaplastic round cells with scant cytoplasm and readily identifiable mitotic activity. These areas had very little stroma, and much fewer lipoblasts (image #4).
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Diagnosis: High grade myxoid / round cell liposarcoma
Myxoid and round cell liposarcomas, although histologically different, actually represent two ends of a spectrum for a tumor which almost always has the same translocation, t(12;16)(q13;p11). They are common, and together account for 50% of all liposarcomas. Tumors with pure myxoid features are considered to be well differentiated, and to have a good prognosis. However, round cell features are considered high grade or poorly differentiated, and are associated with a poorer outcome, even if they represent as little as 5% of the tumor volume. Thus, apparent myxoid liposarcomas should be sampled thoroughly to ensure that no significant round cell component is present. This is a particular problem with needle biopsies, which may entirely miss the round cell component.
Myxoid/round cell liposarcomas frequently metastasize, based in part on the percentage of round cells, with rates varying from 23% (0-5% round cells) to 58% (>25% round cells). They metastasize to lung and bone, as well as to other soft tissue sites.
Histologically, pure myxoid liposarcomas resemble developing fetal adipose tissue, with a multinodular mass of low cellularity, particularly centrally. The tumor cells are bland, spindled or round, and lie within a myxoid matrix. Numerous lipoblasts are present, with one or many vacuoles. There are no/rare mitotic figures. A delicate plexiform vasculature is present, and helps differentiate this tumor from myxoma.
The round cell component is often separated from the myxoid component by a cellular transition area. The round cells are primitive, with minimal eosinophilic cytoplasm, a high N/C ratio, and prominent nucleoli. Lipoblasts are less frequent than in myxoid areas. Often the cells are packed together with minimal stroma, and blood vessels are difficult to find.
As noted, almost all myxoid/round cell liposarcomas share a common t(12;16)(q13;p11) translocation resulting in the TLS-CHOP fusion gene. Less common is t(12;22)(q13;p11) or an insertion between chromosomes 12 and 16, (12;16)(q13;p11.2p13).
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