Parosteal Osteosarcoma

Areas of chondroid differentiation are seen in about half of the cases of parosteal osteosarcoma. In some of these cases, the cartilage is present at the periphery of the lesion and even has a plate-like arrangement simulating an osteochondroma. However, in parosteal osteosarcoma, the chondrocytes are irregularly arranged instead of the columnar arrangement seen in osteochondroma. In addition, the bony trabeculae in parosteal osteosarcoma are separated by a spindle cell proliferation (instead of bone marrow seen in osteochondromas).