CD99 positive adamantinoma of the ulna with ipsilateral discrete osteofibrous dysplasia.

An adamantinoma is a rare, low-grade malignant, osteolytic bone tumor occurring predominantly in the diaphysis of the tibia. Osteofibrous dysplasia has been suggested as a precursor lesion to adamantinoma. Evidence for the relationship between these two tumors is based on their similar histologic features, immunohistochemistry, shared clonal abnormalities, overlapping skeletal distribution, and simultaneous occurrence in the tibia and fibula. The ulna is an unusual site of involvement by adamantinoma and osteofibrous dysplasia. Simultaneous involvement of the ulna by adamantinoma and ossifying fibroma has not been previously reported. A case is presented of an adamantinoma of the distal ulna with unique pathologic features occurring with an ipsilateral discrete focus of osteofibrous dysplasia as additional evidence of the relationship between these two lesions.