Atypical and malignant variants of ossifying fibromyxoid tumor. Clinicopathologic analysis of six cases.

Ossifying fibromyxoid tumor (OFMT) of soft parts is a recently defined fibro-osseous neoplasm, the biologic behavior of which is generally regarded as benign. We report six variant cases of OFMT with histologic features of malignancy, two of which behaved aggressively. All these tumors arose in the extremities of adults (aged 36-76 years), and five of the six were subcutaneous. Four patients were men. Macroscopically, all the tumors were well circumscribed and somewhat lobulated. Cardinal morphologic features included lobules of round to spindled cells within a fibromyxoid matrix and randomly distributed, often centrally located osteoid within which were plump neoplastic cells. In contrast to typical OFMT, a hypocellular, cytologically benign, lamellar bony shell was observed only focally; cellularity was increased (four cases), and mitotic activity was frequent, exceeding two mitotic figures per 10 high-power fields (three cases). One case associated with metastases was morphologically bland. Immunohistochemically, positivity for S-100 protein was observed in the primary tumors of three cases and in the pulmonary metastasis of a fourth. Desmin was positive in one case. Ultrastructural features in three cases were very similar to usual OFMT. Clinical follow-up revealed local recurrence in two cases; one patient has developed recurrent pulmonary metastases. We believe these findings support the view that some atypical cases of OFMT exhibit morphologic patterns that might predict more aggressive behavior.