t(11;18)(q21;q21)-positive advanced-stage MALT lymphoma associated with monoclonal gammopathy: resistance to rituximab or rituximab-containing chemotherapy.

Here we describe two cases of mucosa-associated lymphoid tissue (MALT) lymphoma with monoclonal immunoglobulins (Igs). The first case was a 77-year-old man with primary lymphoma of the lung. Immunoelectrophoresis detected IgM-kappa in serum and kappa light chain excretion into urine. Three months after treatment with single-agent rituximab, a large amount of pleural fluid was found to have accumulated. The fluid contained CD5(-), CD10(-), CD19(+), CD38(+) and CD138(-/+) lymphoma cells with lymphoplasmacytoid appearance. Although a small fraction of the cells were CD20(+), the majority of the lymphoma cells were negative and expressed surface-membrane IgM-kappa at low levels. The cells possessed a karyotype of 46, XY, t(11;18)(q21;q21). The second case was a 55-year-old man who underwent total gastrectomy due to gastric perforation. Surgical specimens demonstrated the histopathological features of MALT lymphoma associated with plasma cell differentiation. The lymphoma cells had a 46, XY, t(11;18)(q21;q21) karyotype. Monoclonal Igs detected were serum IgA (M)-kappa and urinary kappa light chain. The patient was subsequently treated with six cycles of R-CVP (rituximab, cyclophosphamide, vincristine and prednisolone) ; however, serum monoclonal Ig levels were not affected. The lymphoma cells in both cases may have contained two populations, a rituximab-sensitive CD20(+) population and a rituximab-resistant population that had differentiated into the Ig-secreting plasma cell stage.