Bone cancer. Coloured computed tomography (CT) scan showing a left knee (right of image) affected by osteosarcoma. This is a malignant tumour of bone that spreads rapidly to the lungs and, less commonly, to other areas. The first symptom is a painful, visible swelling (blue at centre right) of the affected bone. It occurs mainly in adolescents and the elderly. In young people it develops for no known reason; in elderly people it is a complication of Paget's disease (disruption of bone formation). It is sometimes treated by radiotherapy, but it is usually necessary to remove the affected bone surgically. About half of the patients diagnosed early are cured.