De novo malignant transformation of giant cell tumor of bone

Two cases of malignant lesions are reported, both of which arose secondary to an originally benign giant cell tumor (GCT) of bone. The first case was a typical benign GCT, which occurred in the left proximal tibia of a 31-year-old woman. The tumor was treated by curettage and bone grafting. However, it recurred twice during 15 years of follow-up. The second recurrence showed that the lesion histologically had turned into malignant fibrous histiocytoma. The second case also started as an initially benign GCT that arose in the left distal femur of a 41-year-old man. The patient underwent curettage and bone grafting. The lesion recurred 13 years postoperatively. The histological appearance of the recurrent tumor showed it to be an osteogenic sarcoma. In both patients, radiation had never been given. Malignant transformation has rarely been reported in patients with GCT of bone who have not received radiation treatment.