"Dedifferentiated" chordoma. A clinicopathologic and immunohistochemical study of three cases.

Three cases of "dedifferentiated" chordoma arising in the sacrococcygeal region are presented. In all three cases, the "dedifferentiated" component arose de novo in conjunction with conventional chordoma. Two of these patients, whose tumors had a prominent malignant fibrous histiocytoma (MFH) component, died within 6 months of diagnosis. Both patients had lung metastases, one of which was histologically documented to be MFH. The third patient, whose initial tumor contained osteosarcoma, died 76 months after diagnosis and multiple recurrences. Most notable in this case was the absence of the "dedifferentiated" component (in this instance, osteosarcoma) in all of the local recurrences as well as the lung metastases. These were composed exclusively of conventional chordoma. None of the patients had a previous history of radiation therapy. The immunohistochemical staining pattern of conventional chordoma was similar to that of previous reports, where the epithelial-like cells stained for cytokeratin and epithelial membrane antigen. In addition, they stained for alpha-1-anti-chymotrypsin and vimentin. These latter two markers were also identified in the "dedifferentiated" component. As with "dedifferentiated" chondrosarcomas and liposarcomas, "dedifferentiation" in a chordoma usually portends an accelerated clinical course.