Mucosa-associated lymphoid tissue lymphoma of the thyroid with abundant IgG4-positive plasma cells.

A case of thyroidal mucosa-associated lymphoid tissue (MALT) lymphoma mimicking IgG4-related disease is described. A 54-year-old male presented with acute swelling of the anterior neck. Anaplastic thyroid carcinoma (ATC), malignant lymphoma (ML), or acute deterioration of Hashimoto's thyroiditis were initially suspected, and an emergent tracheostomy was required for progressive airway obstruction; a simultaneous biopsy from the thyroid tissue was performed. Histopathologically, the lesion consisted of sclerotic fibrosis and diffuse and dense infiltration by small lymphoid cells without atypia and plasma cells, many of which were IgG4-positive. Blood examination also revealed high serum IgG4 levels. Riedel's thyroiditis was suspected. However, despite medical treatments, a firm swelling of the thyroid still remained. In an in situ hybridization study, IgG4-negative plasma cells showed immunoglobulin light-chain restriction (κ-monotype), and immunoglobulin heavy (IgH) chain gene monoclonal re-arrangement was detected by polymerase chain reaction. The lesion was finally diagnosed as MALT lymphoma. When IgG4-related disease is suspected, it is important to thoroughly exclude other possibilities.