Maltoma of the thyroid in a man with Hashimoto's thyroiditis.

We report the case of a 42-yr-old man with primary thyroid lymphoma arising from mucosa-associated lymphoid tissue (MALT-lymphoma, maltoma). The patient underwent a hemithyroidectomy for a growing mass in the right lobe of the thyroid while being treated with 1-thyroxine for Hashimoto's thyroiditis. The clinical diagnosis of Hashimoto's disease was confirmed by aspiration biopsy of the mass during the course of L-thyroxine treatment. Postoperatively, histology showed atypical lymphoproliferative infiltrates suspicious of low-grade non-Hodgkin's lymphoma of mucosa-associated lymphoid tissue-type, coexisting with a reactive process typical of chronic lymphocytic thyroiditis. Immunophenotyping showed a mixed B- and T-lymphocyte population, which was nondiagnostic. However, Southern blot analysis revealed a clonal rearrangement of the Ig heavy chain gene. This case demonstrates that cytology or histology may not distinguish between reactive or low-grade lymphomatous thyroid processes. The use of molecular technique was essential to prove clonality and the presence of lymphoma.