Parosteal osteosarcoma

From Felipedia

Parosteal osteosarcoma is the second most common tumor in the cat and arises on the outer surface of the cortex of bones. Such tumors have been reported to arise from the humerus and femurs and from the frontal bone and ramus of the mandible. These tumors grow slowly and cause increasing lameness and inability to use the affected limb. Affected animals range in age from 1 year to 14 years, with a mean age of 6.6 years[1].

Grossly the tumors occur adjacent to the periosteum, but no cortical involvement is found in early cases. However, in long-standing cases, these tumors may invade the cortex and extend into the medullary cavity. There may be extension into the adjacent muscle tissue. Histologically, there is proliferation of fusiform cells resembling fibroblasts, with production of chondroid and osseous foci. Cellular pleomorphism and mitotic activity are found infrequently.

Parosteal osteosarcomas do not often metastasize. Thus, surgical amputation of the affected limb is usually curative. We have seen a single case of lung metastasis following cortical and medullary invasion.