Thyroid low-grade B-cell lymphoma (MALT type) with extreme plasmacytic differentiation: report of a case diagnosed by fine-needle aspiration and flow cytometric study.

We describe a case of thyroid marginal-zone B-cell lymphoma of mucosa-associated lymphoid tissue (MALT lymphoma) with extreme plasmacytic differentiation in an 80-year-old woman who had presented with a rapidly growing thyroid swelling. The diagnosis was initially suspected on fine-needle aspiration (FNA) and subsequently confirmed by flow cytometric analysis of the aspirated material. The smears revealed features of Hashimoto's thyroiditis admixed with large numbers of atypical large plasmacytoid lymphoid cells accompanied by variable numbers of small lymphocytes and mature plasma cells. The differential diagnosis of a predominantly plasmacytic smears on FNA of the thyroid is discussed herein, with emphasis on the role of immunophenotypic studies to exclude or confirm the diagnosis of lymphoma.