Small Lymphocytic Lymphoma of the Thyroid Mimicking Plasmacytoma

Primary thyroid gland lymphomas (PTLs) typically occur in middle-to older-aged individuals in the setting of lymphocytic thyroiditis with a predilection for females. Diffuse large B-cell lymphoma is the most frequent histologic subtype of thyroid lymphomas. Small lymphocytic lymphoma (SLL) belongs to the least common subtypes of thyroid lymphoma. It is often associated with the involvement of lymph nodes, bone marrow, spleen, liver and, extremely rarely, other organs. PTLs with plasmacytic differentiation or extensive infiltration by plasma cells have been observed in marginal zone B-cell lymphomas in the thyroid but have never been described in a setting of SLL. Here, we present a case of primary SLL of the thyroid mimicking extramedullary plasmacytoma on fine-needle aspiration cytology.