Immunohistochemical, electron microscopic, and immunoelectron microscopic features of plasmacytoma of the thyroid with amyloid deposition.

A rare case of primary plasmacytoma of the thyroid with amyloid deposition is reported. The tumor consisted of diffuse proliferation of atypical plasma cells showing IgG lambda-type monoclonal growth. Amyloid deposition with focal giant cell reaction was also observed. Bone marrow aspiration and systemic skeletal radiographic surveys revealed no evidence of multiple myeloma and myelomatosis. Radial arrangement of amyloid bundles was observed ultrastructurally. By immunoelectron microscopic examination, lambda chain was detected in the amyloid fibrils and rough endoplasmic reticulum of the tumor cells. Our findings suggest the following: amyloid fibrils originate from monoclonal light chains produced by tumor cells, and histiocytes contribute to amyloid deposition as well as to giant cell reaction in extramedullary plasmacytoma.