Answer: Mesenchymal chondrosarcoma

Histology: The largest series of these lesions has been published from the Mayo Clinic by Nakashima and colleagues [1]. About two thirds arise in bone and the remainder in soft tissue. In skeletal sites, the mandible, rib, pelvic bones, and femur are the most common sites whereas extraskeletal examples presented in lower extremities followed by the meninges. Most lesions present in the second and third decades of life. Central nervous system examples have been reviewed by Rushing et al [2] and Vencio has called attention to the fact that, while these are generally aggressive tumors, examples arising in the jaw bones can pursue a more indolent course [3]. Their histologic features are usually diagnostic without ancillary studies as these tumors consist of “small round cells” with scant cytoplasm arranged around hemangiopericytoma-like vasculature and punctuated by islands of mature-appearing cartilage. Reticulin staining encloses small groups of cells and ultrastructure of the cartilage areas discloses well-developed golgi, glycogen, and rough endoplasmic reticulum (features in keeping with cartilaginous differentiation) whereas the small cells show minimal organelles (undifferentiated features).

Discussion: When the cartilage islands are not well represented or when the cells assume a spindled pattern, diagnosing mesenchymal chondrosarcoma can be more difficult. Immunohistocemistry can be helpful in such cases, which can bear a striking resemblance to both solitary fibrous tumor and to poorly differentiated synovial sarcoma. These tumors are CD99 (013) positive [4] and show S100 in the zones of cartilage as well as in scattered small cells [2]. They lack actins, desmin, and epithelial membrane antigen (synovial sarcoma usually has focal epithelial membrane antigen). CNS examples sometimes have GFAP and focal keratin. As an aside, CD99 has been found in several tumors in the differential diagnosis (poorly differentiated synovial sarcoma, solitary fibrous tumor, and hemangiopericytoma)[5], so a panel approach is always warranted.