Primary marginal zone lymphoma of the thymus

Primary low-grade B-cell lymphomas of the thymus are rare, with only 7 repo rted cases in the literature. We describe 3 cases of primary low-grade thym ic lymphoma. All had histologic features of extranodal marginal zone lympho ma and were composed predominantly of small lymphocytes with variable compo nents of monocytoid cells and plasma cells. Overt transformation to large c ell lymphoma occurred in 1 case. The neoplastic cells were immunoreactive f or the B-cell marker CD20 and were positive for bcl-2 in 2 cases. Two of 3 patients had a long-standing history of autoimmune disease. Based on these findings and those of previously reported cases, marginal zone lymphoma is the predominant type of low-grade thymic B-cell lymphoma. These tumors seem to be more common in patients with autoimmune disorders, and as observed w ith marginal zone lymphoma arising at other anatomic sites, they may underg o transformation to a higher grade lymphoma.