Describe periosteal osteosarcoma.

Periosteal osteosarcoma, which arises from the deep periosteum, accounts for only 2% of osteosarcomas and is usually seen in young patients (peak age, 10-20 years).

Periosteal osteosarcoma tends to appear as a sessile, elliptic, or saucer-shaped cortical depression with aggressive periostitis and without involvement of the medullary space. There is little soft tissue mineralization.