Low-grade B-cell lymphoma of mucosa-associated lymphoid tissue (MALT) of thymus

This report describes a low-grade B-cell lymphoma of mucosa associated lymp hoid tissue (MALT) involving the thymus of a 63-year-old woman with feature s suggestive of a connective tissue disease. Sections of the thymic lesion and of a lung biopsy performed at the same operation were examined histolog ically and by immunohistochemistry using the monoclonal antibodies CD45, CD 20, CD79a, CD3, CD45RO, and AE1/AF3, Polymerase chain reaction (PCR) for im munoglobulin heavy chain gene rearrangement was also performed. The dense i nfiltrate of small lymphoid cells intimately admired with ramifying epithel ial elements, some of which had undergone cystic change, closely resembled a thymoma. The lymphoid infiltrate comprised centrocyte-like cells, small l ymphocytes, plasma cells, and blasts. Most of the lymphoid cells were immun oreactive with the E-cell markers CD20 and CD79a, and PCR showed clonal imm unoglobulin heavy chain gene rearrangement. The lung biopsy showed dense in filtration by small lymphoid cells, morphologically suggestive of lymphoid interstitial pneumonia, However, PCR showed a weak band in the amplificatio n for immunoglobulin heavy chain gene rearrangement, identical to that with in the thymus and suggesting either recirculation of cells to accumulated M ALT or sub-histological lymphoma. MALT lymphoma may rarely involve the thym us, and pathologists should be aware of this to avoid misdiagnosis as a thy moma Immunohistochemical and/or molecular studies are of value in this rega rd. MALT lymphomas of the thymus, common with those arising in other organs , may develop in the setting: of a connective tissue disease. Copyright (C) 2000 by W.B. Saunders Company.