Extranodal marginal zone B cell lymphoma of MALT type with extensive plasma cell differentiation in a man with Hashimoto's thyroiditis.

Primary thyroid gland lymphomas are uncommon tumours that occur in the setting of lymphocytic thyroiditis or Hashimoto's disease in almost all cases. Rarely these tumours show extensive plasmacytic differentiation. In these conditions, a distinction between an inflammatory thyroid infiltrate and extramedullary plasmacytoma may be extremely difficult and precise criteria and ancillary techniques are necessary for a correct diagnosis. The authors report a case of mucosa associated lymphoid tissue (MALT) lymphoma of the thyroid gland in a 69-year-old Iranian man with Hashimoto's thyroiditis which was diagnosed by means of immunohistochemical stains. This case demonstrates that histology may not distinguish between extramedullary plasmacytoma and MALT lymphoma of the thyroid gland and the use of immunohistochemical staining stains were essential to prove definite diagnosis.