![]() ![]() However the t(8 8) fusion can be cryptic and may not be seen in all the karyograms. Most of the published cytogenetic studies revealed diverse karyotypic findings, until recently Wang et al 21 identified a novel, recurrent HEY1-NCOA2 (8 8)(q21 q13) fusion in MCS. 19, 20 Shakked et al 6 previously reported a complete list of the published cytogenetic findings. 18 Others have reported isolated cases with trisomy 8 as well as reciprocal translocation (11 22)(q24 q12). A 13 21 Robertsonian translocation has been described in 2 cases, 1 each of skeletal MCS and EMCS. Most of the EMCSs studied in the past were found to have complex cytogenetic alterations. 17 Cytogenetics and Molecular Diagnostics 13 INI-1 expression can also be useful in distinguishing EMCS from other soft tissue tumors with loss of INI-1, such as epithelioid sarcoma, myoepithelial carcinoma, and extraskeletal myxoid chondrosarcoma. 16 The tumor cells retain expression of INI-1 immunostain, and hence it can be used to distinguish EMCS in the central nervous system from atypical teratoid rhabdoid tumor. The tumor is also negative for FLI-1, which can be helpful in excluding MCS and identifying Ewing sarcoma when only small, round blue cells are present in the biopsy specimen. In general, there is no single best stain for this tumor, and hence a panel of stains should be used to arrive at an accurate diagnosis. 15 However, it cannot be used as a sensitive and specific marker for this tumor. 13, 14 Type II collagen stains the extraskeletal matrix of the small cell regions in EMCS, and has been shown to be useful in distinguishing EMCS from other round cell sarcomas in a small series of limited tumor types. However, rare cases showing scattered tumor cells with desmin, myogenin, and myoD1 positivity have also been described in tumors with areas of rhabdomyosarcomatous differentiation. ![]() 9, 12 Other markers, including cytokeratin, epithelial membrane antigen, and muscle markers, are usually negative. Sox9 is a recently described marker that shows nuclear positivity in both undifferentiated mesenchymal cells and cartilaginous component ( Figure 6). As with other round cell tumors, the undifferentiated cells are positive for CD99 (strong membranous), neuron-specific enolase, and Leu-7, a fact limiting use of the previous immunostains. The undifferentiated cells show scant patchy positivity for S100 protein. Similar to other chondrosarcomas, the cartilaginous component is strongly positive for S100 protein. The tumor is diagnosed primarily on the basis of morphology because of a nonspecific immunoprofile and a general lack of availability of molecular diagnostics. The tumor has a characteristic biphasic histology comprising sheets of primitive mesenchymal cells with interspersed islands of well-differentiated hyaline cartilage. It is a high-grade malignancy with strong tendency for distant metastasis. Extraskeletal MCS represents about 1% of all chondrosarcomas. The first case of MCS occurring at an extraskeletal site was described 5 in 1964. More recent literature 3, 4 reports that 20% to 33% of these tumors occur at extraskeletal sites. Originally, it was considered restricted to bone, but that is no longer the case. Two years after its original description, Dahlin and Henderson 2 reported 9 cases from the files of the Mayo Clinic. Mesenchymal chondrosarcoma (MCS) was first described by Lichtenstein and Bernstein 1 in 1959. Extraskeletal mesenchymal chondrosarcoma (EMCS) is a rare malignant soft tissue tumor of chondroprogenitor cell origin.
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