Follicular lymphoma of the thyroid gland.

The majority of lymphomas arising in the thyroid gland are mucosa-associated lymphoid tissue lymphomas and diffuse large B-cell lymphomas, which arise from a background of chronic lymphocytic thyroiditis. Follicular lymphoma may also present in the thyroid gland, but its clinicopathologic features at this site are not well characterized, leading to difficulties in diagnosis and clinical management. We have addressed this problem by studying the clinical, morphologic, immunophenotypic, and genetic features of 22 such cases. All cases showed morphology characteristic of follicular lymphoma, however, in many the interfollicular neoplastic infiltrate was particularly prominent and all lymphomas contained readily identifiable and often striking lymphoepithelial lesions, features heretofore considered indicative of mucosa-associated lymphoid tissue lymphoma at this site. Furthermore, 13 of 18 cases for which sufficient evidence was available had clinical and/or histologic evidence of chronic lymphocytic thyroiditis. Analysis of genetic and immunohistochemical features identified 2 distinct groups. In 1 group, similar to typical adult follicular lymphoma, cases carried a t(14;18)/IGH-BCL2 and/or expressed Bcl-2, and were mostly CD10-positive and of World Health Organization (WHO) grade 1 to 2. Follicular lymphomas in the other group lacked IGH-BCL2 and Bcl-2 expression, were often of WHO grade 3 and were often CD10-negative, similar to the minority of follicular lymphomas previously described that are Bcl-2-negative and are often encountered at other extranodal sites. The 2 groups differed in clinical stage at presentation, 11 patients in the former group but none in the latter group having disease beyond the thyroid gland. Appreciation of the spectrum of morphologic, immunophenotypic, and genetic characteristics of follicular lymphoma presenting in the thyroid gland should aid both diagnosis and clinical management.