Parosteal osteosarcoma of the distal ulna. A rare tumour at a rare location: a case report.

We report a case of a histologically well differentiated, grade I, parosteal osteosarcoma of the distal ulna. The tumour's radiological and histological features are described, and the patient's course after wide en-bloc resection is detailed. Parosteal osteosarcoma is a rare low-grade malignant neoplasm. It belongs to the group of the osteosarcomas originating from the surface of the bone, representing a distinct tumour entity within this group, with defined histological and radiological features. We describe the radiological and histological characteristics of parosteal osteosarcoma and, based upon these features, present its classification. The differential diagnosis, including parosteal osteoma, myositis ossificans and osteochondroma, is also discussed. Finally, therapy and prognosis are outlined in accordance with the current literature. Wide en-bloc resection represents the mainstay of therapy offering an excellent prognosis with a 5-year survival rate of more than 90%. Incomplete resection, on the other hand, increases the risk of recurrence and dedifferentiation of the tumour. Dedifferentiation induces tumour spread and is associated with a poor prognosis equivalent to conventional osteosarcoma.