Fine needle aspiration cytology and immunocytochemistry in the diagnosis of lymphoid lesions of the thyroid gland.

Immunocytochemical analysis was used in conjunction with cytomorphology to evaluate fine needle aspirates from 18 patients with nodular lymphoid infiltrates of the thyroid. The smears from 13 patients had a cytomorphology that was diagnostic of chronic lymphocytic thyroiditis; immunocytochemistry showed the lymphoid population to be composed of T cells and polyclonal B cells. Three patients had high-grade malignant lymphomas by cytomorphology; immunologic evaluation showed the B-cell phenotype of the lymphoma cells and monoclonal light chain expression. Cytomorphology could not differentiate between an inflammatory and a neoplastic lymphoid infiltrate in two cases. Immunocytochemical analysis showed that the cells of B-cell phenotype were monoclonal (neoplastic) in one case and polyclonal (inflammatory) in one case.