Malignant lymphoma of the thyroid gland with plasma cell differentiation (plasmacytoma).

An unusual case of malignant lymphoma of the thyroid is reported. The predominant appearance of the neoplasm was that of a large cell ("histiocytic") lymphoma, but a discrete portion of the tumor showed features typical of plasmacytoma. Electron microscopy revealed a wide spectrum of cell types, ranging from very immature lymphoid cells to relatively well-developed plasma cells. The findings support the concept that the neoplastic process represents an immunoblastic sarcoma composed of proliferating transformed B lymphocytes (immunoblasts) and their progeny (plasma cells) rather than two distinct lymphoid neoplasms occurring simultaneously.