sarcoma osteosarcoma (disease) 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. DOID:3347 EFO:0000637 HP:0002669 ICDO:9180/3 MONDO:0009807 NCIT:C9145 ONCOTREE:OS bone tissue neoplasm osteogenic sarcoma osteoid sarcoma osteosarcoma osteosarcoma, malignant sarcoma of osteoid skeletal sarcoma