LOW-GRADE B-CELL LYMPHOMA OF MUCOSA-ASSOCIATED LYMPHOID-TISSUE IN THETHYMUS OF A PATIENT WITH RHEUMATOID-ARTHRITIS

The majority of thymic lymphomas are either lymphoblastic lymphoma, la rge B cell lymphoma or Hodgkin's disease, and other types of non-Hodgk in lymphoma are rare. A case of low-grade B cell lymphoma of mucosa-as sociated lymphoid tissue (MALT) in the thymus is reported. A 55-year-o ld Japanese female with a history of rheumatoid arthritis (RA) complai ned of back pain. A mediastinal tumor was identified by computerized t omography and magnetic resonance imaging, and the thymus was resected through median sternotomy. The solid and nodular tumor had several sma ll satellite extensions and was completely confined to within the thym us. Histologically, monotonous medium-sized centrocyte-like cells occu pied the medulla of the thymus and infiltrated Hassall's corpuscles (l ymphoepithelial lesions). Immunohistochemically, tumor cells were posi tive for CD20 and CD79a. IgA and kappa light chain restriction were al so found in plasmacytoid cells in the tumor. Clonal rearrangement of t he immunoglobulin heavy chain gene was demonstrated by polymerase chai n reaction. This case was diagnosed as MALT-type low-grade B cell lymp homa in the thymus. This is the first report of low-grade B cell lymph oma in the thymus associated with RA. As autoimmune diseases are known to be associated with lymphoid neoplasms, it is suggested that the RA played an important role in the development of malignant lymphoma in this case.