MALT Type Lymphoma Demonstrating Prominent Plasma Cell Differentiation Resembling Fibrous Variant of Hashimoto's Thyroiditis: a Three Case Report

Fibrous variant of Hashimoto's thyroiditis (HT) is characterized by marked fibrous replacement of one third or more of the thyroid parenchyma. We present here three cases of mucosa associated lymphoid tissue (MALT) type lymphoma demonstrating prominent plasma cell differentiation resembling fibrous variant of HT. Histologically, thyroid structures were disturbed by a diffuse and focally nodular infiltration by mature plasma cells and cells with plasma cell differentiation against a background of prominent hyalinosis. In addition, scattered centrocyte-like (CCL) cells and lymphoepithelial lesions were observed in all three lesions. A portion of the resected specimens in all three cases exhibited HT. However, immunohistochemical study demonstrated that the plasma cells and CCL-cells of these three lesions had monotypic intracytoplasmic kappa light chain. Moreover, these three lesions demonstrated a clonal band on polymerase chain reaction assay for the immunoglobulin heavy chain gene. To avoid underdiagnosis, we emphasize that careful attention should be paid to these immunological features as well as to morphological findings.