Answer: Mesenchymal chondrosarcoma

Histology: The radiologic features in this case show an aggressive lytic lesion. There is no mineralization present. This lack of mineralization is atypical for a mesenchymal chondrosarcoma, a lesion which frequently shows a ring and stipple-like pattern of mineralization. Histologically, there are two components to mesenchymal chondrosarcoma. One component is a population of small round blue cells, which may be spindled in areas. Very often this component exhibits a hemangiopericytoma-like pattern. The second component is hyaline cartilage, which is uniformly and intimately admixed with the tumor cells.

Discussion: A dedifferentiated chondrosarcoma also has two components to its histology. One component is hyaline cartilage and the other is a high-grade sarcoma, either malignant fibrous histiocytoma or an osteosarcoma. However, unlike mesenchymal chondrosarcoma where the components are mixed uniformly throughout the entire lesion, dedifferentiated chondrosarcoma shows two very distinct populations quite separate and discrete from each other.

Conventional chondrosarcoma lacks the small round hemangiopericytoma-like areas of mesenchymal chondrosarcoma. This lesion shows a rather homogeneous population of lobules of atypical cartilage cells.

A small round blue cell tumor, such as Ewing’s tumor, would lack the lobules of cartilage that are present in a mesenchymal chondrosarcoma.