Primary malignant transformation of giant cell tumor of bone

The malignant transformation of giant cell tumor (GCT) of bone is a relatively rare event. Most cases are ‘secondary’ tumors, usually arising in lesions that have been treated previously with radiotherapy. Less commonly, secondary malignancy occurs in GCTs that have been treated surgically. The growth of ‘primary’ malignant GCTs has also been reported, but with much lower frequency. In both primary and secondary malignant GCTs, osteosarcomatous transformation is the most frequent histological type seen however malignant fibrous histiocytoma and fibrosarcoma may also develop.Currently, there are no proven radiological or pathological features that can reliably be used to predict malignant transformation in GCT of bone. We discuss the case of a primary malignant GCT with intratumoral calcification. This radiographic finding may herald the presence of osteosarcoma.