Chondrosarcomas are polarized by age and location. In the older patient population, they occur primarily in long tubular bones, whereas in the younger population they occur more often in flat bones. The current histologic grading scheme separates these tumors into 3 grades based on cellularity, cytologic atypia and mitotic activity. Compared to enchondromas, grade 1 chondrosarcomas have increased cellularity and contain plump nuclei with more open, vesicular chromatin. The cells tend to be of uniform size and binucleation is usually present. With increasing grade, chondrosarcomas become more cellular and develop large hyperchromatic nuclei with increasing degrees of nuclear atypia and mitotic activity.
Dedifferentiated chondrosarcomas have a malignant cartilaginous component that is juxtaposed with a divergent, high grade spindle cell sarcoma. These two cell populations are sharply demarcated, with no transition zone between them. In the case presented above, the cartilaginous component was represented by a grade 2 chondrosarcoma. Interestingly, the dedifferentiated component had features of an osteosarcoma, with true malignant osteoid formation. Johnson et al. reviewed a series of 26 cases of dedifferentiated chondrosarcoma and found that only 2 cases showed osteosarcomatous differentiation in the high grade spindle cell component. More commonly, this component showed a histologic appearance of fibrous or myoid differentiation.
The differential diagnosis for dedifferentiated chondrosarcoma is potentially extensive depending on the features of the dedifferentiated component and the location of the lesion. In this case, the differential diagnosis would include mesenchymal chondrosarcoma. Microscopically, mesenchymal condrosarcoma has a biphasic pattern of undifferentiated small round cells admixed with islands of hyaline cartilage, this could look microscopically similar to dedifferentiated chondrosarcoma. However, it will not show a sharp demarcation between components. In addition, the undifferentiated areas of mesenchymal chondrosarcoma typically simulate Ewing sarcoma, and a hemangiopericytomatous vascular pattern is common (this vascular pattern is not seen in dedifferentiated chondrosarcoma). The most common sites for mesenchymal chondrosarcoma include craniofacial bones, vertebrae, ilium, and ribs.
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Answers: 1C (a flat bone), 2A, 3C, 4B, 5D
Contributed by Kate Serdy, MD and Alka Palekar, MD