A usually aggressive malignant bone-forming mesenchymal neoplasm, predominantly affecting adolescents and young adults. It usually involves bones and less frequently extraosseous sites. It often involves the long bones (particularly distal femur, proximal tibia, and proximal humerus). Pain with or without a palpable mass is the most frequent clinical symptom. It may spread to other anatomic sites, particularly the lungs. The disease is MONDO_0009807 (osteosarcoma). Also known as: osteogenic sarcoma, osteoid sarcoma, osteosarcoma (disease), osteosarcoma, malignant, sarcoma of osteoid.