Osteosarcoma is a type of bone cancer that most commonly occurs in the long bones of the body, particularly in the area around the knee. It is more commonly diagnosed in adolescents and young adults.

The pathology of osteosarcoma is characterized by the presence of malignant bone-forming cells that produce osteoid tissue (bone matrix without mineralization) and tumor giant cells. The neoplastic cells have a high rate of division and eventually form a mass (tumor) that destroys the normal bone structure. The resulting destruction of the normal bone architecture and formation of new, abnormal bone can be seen on X-rays and other imaging studies.