Myxoid variant of follicular dendritic cell sarcoma arising in the breast.

Follicular dendritic cell sarcoma is a malignant tumor of the follicular dendritic cell which can arise in extranodal sites. We present here a case arising as a mass in the breast of a 41-year-old woman. The tumor was composed of mildly pleomorphic spindly cells with pale ovoid nuclei and cell processes intimately admixed with mature lymphocytes. In much of the lesion the cells were dispersed in cords in a myxoid stroma, and elsewhere there were solid sheets. The neoplastic cells were immunoreactive for CD21, CD35, EMA, and S100 protein, but not for other lymphoid markers or cytokeratin. Electron microscopy showed interdigitating cytoplasmic processes with junctions but no external lamina. The differential diagnosis includes carcinoma, lymphomas, and a variety of myxoid sarcomas. The tumor recurred within a few months and displayed increased nuclear pleomorphism and lymphatic invasion but the patient appears free of disease 3 years after the further excision. This case extends the spectrum of follicular dendritic cell sarcoma in soft tissue sites.