Osteosarcoma

The osteosarcoma is the commonest of the primary malignant bone tumours. Its peak incidence is before epiphyseal fusion, particularly the years between 10 -25. There is a later peak which is associated with Paget's disease or previous radiotherapy. Visible Paget's disease does not have to be present before one can make the diagnosis, however.

50% cases are related to the knee, effecting the metaphyses and adjacent diaphyses of femur and tibia. They seem to arise in the long bones which show the greatest longitudinal growth. With large tumours, the site of origin is inferred from the centre of radius of the mass. The diaphyses are involved more extensively than plain films indicate. Joint involvement is rare.

The appearance is of an eccentric destructive lesion with more or less irregular bone formation. 5% of tumours will be purely lytic. Typically, the soft tissue mass will breach the cortex and elevate the periosteum with new bone formation giving rise to the Codman's triangle. The soft tissue mass may contain spiculated bone or amorphous calcification. Most soft tissue sarcomas of bone are capable of metaplasia so this can confuse the appearance. The tumour comprises osteoblasts and its particular characteristic is bone formation. The tumour metastasises early. There are some variants in the appearance in that an essentially bone forming, more central osteosarcoma may superficially resemble a large bone island. Even with a purely lytic lesion, nearly a third of bone has to be destroyed before a lucency is visible on the plain films.

There is an entity known as multifocal osteosarcoma which is only found in childhood and is a very aggresive tumour with marked elevation of the alkaline phosphatase.