Parosteal Sarcoma.

This tumour has an onset later than osteosarcoma. 50% of cases are over 30 yrs old. It comprises 4% of all osteosarcomas. The commonest site is in the femur or tibia, although the humerus can be involved. The tumour may be large and may affect between 2 - 10 cms. of bone.

Its appearance is of a dense bone forming tumour and has an undulating appearance with a well defined margin. There should be a plane of cleavage between the density of the tumour and the density of the cortex of the long bone. Demonstration of this may be difficult and may require oblique views, tomography or CT. The tumour is attached by a broad pedicle to the underlying bone. The tumour is malignant, but is usually slower growing than osteosarcoma. It metastasises a little later than osteosarcoma.