The significance of bone and cartilage formation in malignant fibrous histiocytoma of soft tissue.

Malignant fibrous histiocytoma of soft tissues (MFH), a mesenchymal tumor of varied morphologic patterns and cell types, sometimes contains bone and cartilage. Such bony and cartilaginous elements in a pleomorphic MFH may pose a difficult diagnostic challenge. An MFH with bone and cartilage can be distinguished from extraosseous osteogenic sarcoma and chondrosarcoma on the basis of qualitative and quantitative features of the osseous and chondroid elements. Five cases of soft tissue MFH containing bone and cartilage reported here showed: 1) that the bony and cartilaginous elements are mostly metaplastic and tend to be in the pseudo-capsule or fibrous septa of the tumor; and 2) that the bony elements may show a zoning pattern with peripheral maturation similar to myositis ossificans. In a single case, there was a small amount of "tumor osteoid." However, this was focal and lacked the diffuse and finely divided or ribbon-like pattern generally associated with osteogenic sarcoma. The probability that MFH with bone and cartilage is less aggressive than soft tissue osteogenic sarcoma or chondrosarcoma underscores the importance of accurate histologic diagnosis of these tumors.