Parosteal osteosarcoma is a distinct well-differentiated subtype with better prognosis than the conventional osteosarcoma. It is located on the outer surface of the cortex and is also referred to as juxtacortical osteosarcoma. It is seen in slightly older patients (2nd - 4th decades of life) and the most commonly affected sites are metaphysis or metadiaphyseal region of femur (especially posterior distal shaft of the femur), humerus, and tibia as shown in this diagram. Slide courtesy of Piero Picci, M.D., Director, Laboratory of Experimental Oncology, Instituto Ortopedico Rizzoli, Bologna, Italy. Used with permission.