parosteal osteosarcoma

a variant of osteosarcoma consisting of a slowly growing tumor that arises from the cortex of a bone and grows outward, eventually surrounding the bone; it resembles cancellous bone and is characterized histologically by a fibrous stroma containing bony spicules and trabeculae. It affects adults in the second through fifth decades of life and is less malignant than classical osteosarcoma. Called also juxtacortical o. and parosteal sarcoma.