Gross specimen of an osteosarcoma of the femur (thigh bone) showing a large destructive tumour that has eroded through the bone cortex and produced a bulky soft tissue mass. The cut surface shows firm, fleshy areas interspersed with foci of haemorrhage and necrosis. Osteosarcoma is a malignant neoplasm in which the tumour cells produce bone. It is the most common primary malignant bone tumour (excluding hematopoietic malignancies). The peak incidence is between the ages of 10 and 25 years and a second peak is seen after the age of 40 years. They usually arise in the metaphyseal area of long bones, especially the femur. Other common locations include proximal tibia and proximal humerus.