Primary malignant giant cell tumor of bone: "dedifferentiated" giant cell tumor.

Well documented examples of primary malignant giant cell tumor of bone (giant cell tumor and concurrent sarcoma arising de novo) are exceedingly rare in the literature. We report a case arising in the left ischium of a 44-yr-old man. He had no previous history of radiation therapy or multiple resections. Histologically, the tumor was a typical giant cell tumor of bone juxtaposed to a malignant fibrous histiocytoma (MFH). The juxtaposition of a high grade sarcoma (MFH) and a locally aggressive nonmalignant neoplasm such as giant cell tumor is analogous to several other tumors of bone and soft tissue in which a low grade malignant or locally aggressive tumor can be associated with MFH or fibrosarcoma de novo, namely chondrosarcoma, chordoma, liposarcoma, and well differentiated intraosseous and parosteal osteosarcoma. The presence of a high grade malignant component in each of the aforementioned neoplasms generally portends a more ominous prognosis, although this is not invariably true. Recognition of the phenomenon of "dedifferentiation" (or tumor progression) in some bone tumors and sarcomas is important to ensure appropriate treatment. Distinction from secondary malignant giant cell tumors which are usually radiation induced is also important, since the latter have a much worse prognosis than those with dedifferentiation occurring de novo.