Benign bone tumours. X-ray of the hands of a patient with benign (non-cancerous) bone tumours called exostoses. One such exostosis can be seen on the left thumb (centre). Exostoses usually produce no symptoms, and are only noticed when they have grown to a prominent size, as here. They are often caused by hereditary factors, and may also arise due to prolonged pressure on a bone. An exostosis may be surgically removed if it presses on an important body structure such as a nerve or blood vessel.