Juxtacortical osteogenic sarcoma is one of many variants of osteogenic sarcoma.  As its name implies, it arises on the surface of the bone, but can invade into the medullar canal.   The term juxtacortical osteosarcoma is used to describe two different sub-types of osteosarcoma, paraosteal and periosteal osteosarcoma.  Paraosteal osteosarcoma is typically (but not always) a low-grade form of osteosarcoma that is radiodense in appearance on x-ray and sits on the cortical surface of bone.  It is often described as appearing like wet cotton balls placed on the bone surface.  It is most commonly seen in the posterior aspect of the knee or in the proximal humerus.  The histology shows wide seams of osteoid in a slightly bland fibrous background.  Periosteal osteosarcoma is also found of the surface of bone, but has a more spiculed appearance and may appear more aggressive.  The histology will show malignant osteoid and malignant osteoblast cells as well as variable amount of cartilaginous matrix.  It is usually an intermediate to high-grade form of osteosarcoma (but low grade histology does exist).   The term juxtacortical osteosarcoma is slowly replacing the older nomenclature of periosteal and paraosteal osteosarcoma because it has become more evident that these terms once used to describe low grade (paraosteal) and intermediate grade (periosteal) no longer adequately describe the true spectrum of tumor biology that exists among surface osteosarcoma.