Mesenchymal chondrosarcoma: A small cell neoplasm with polyphenotypic differentiation

We analyzed the clinicoradiographic, microscopic, and immunophenotypic features of 21 tumors from 13 patients with mesenchymal chondrosarcoma (11 primary and 10 metastatic) and addressed the issue of their potential polyphenotypic differentiation. The immunophenotypic profile of the tumors was analyzed by studying the expressions of the MIC2 gene protein (p30/32^MIC2), S-100 protein, desmin, myoD1, myogenin, myoglobin, smooth-muscle actin, cytokeratin, neuron-specific enolase, and HMB-45. The expression of p30/32^MIC2 was typically restricted to the small cell component of the mesenchymal chondrosarcoma and could be documented in 17 tumors. The cartilaginous areas were positive for S-100 protein in 20 tumors. Scattered positivity of small cells for desmin was seen in 8 cases. In 2 primary tumors from different patients (1 intraosseous and 1 extraskeletal) a diffuse expression of desmin with focal coexpression of myoD1 was present within the small cell component of the tumor. The positivity for smooth-muscle actin was documented in 2 cases. Either the small cell or cartilaginous components were at least focally positive for neuron-specific enolase in 11 tumors. All tumors were negative for myogenin, myoglobin, cytokeratins (AE1/AE3, CAM5.2) and HMB-45. This study showed that, in addition to cartilaginous differentiation, mesenchymal chondrosarcomas may exhibit focal expression of desmin. In rare cases more diffuse rhabdomyoblastic differentiation can be seen within the small cell component of the tumor. Thus, mesenchymal chondrosarcoma is another primitive neoplasm with polyphenotypic differentiation and features that overlap those of other small cell malignances of bone and soft tissue.