label: Periosteal Osteosarcoma
label_lc: periosteal osteosarcoma
description: "An intermediate grade malignant bone-forming mesenchymal neoplasm with chondroblastic differentiation.  It arises from the surface of the bone and affects the diaphysis or diaphyseal- metaphyseal portion of the long bones.  A painless mass or swelling is the most common clinical sign.  It is associated with a better prognosis than conventional osteosarcoma."