B-cell lymphoma of mucosa-associated lymphoid tissue of the thymus: a report of two cases with a background of Sjögren's syndrome and monoclonal gammopathy.

Two rare cases of low-grade B-cell lymphoma of mucosa-associated lymphoid tissue (MALT) arising in the thymus are reported. Both patients (a 61-year-old man and a 75-year-old woman) were suffering from Sjögren's syndrome and immunoglobulin (Ig)A kappa monoclonal gammopathy. Mixed IgA-IgG cryoglobulinemia was also present in the male case. Tumor cells expressed IgA and kappa antibody reactive proteins identical with serum IgA kappa M. Moreover, we could demonstrate rearrangements of the immunoglobulin heavy and light chain genes, which supported the monoclonal origin of tumor cells. Immunological abnormalities improved after thymectomy in one case in which the tumor cells were confined to the thymus, but not the other with regional lymph node involvement, suggesting a causal role for the tumor. MALT lymphomas of the thymus thus appear to be associated with immunological disorders such as Sjögren's syndrome or monoclonal gammopathy.