Transformation of MALT lymphoma to pure plasma cell histology following treatment with the anti-CD20 antibody rituximab.

Mucosa associated lymphoid tissue (MALT) lymphoma is a relatively common lymphoma arising from marginal-zone B-cells which are closely related to plasma cells. As opposed to the large majority of plasma cells, MALT lymphoma cells express CD20, and the anti-CD20 antibody rituximab has been reported as active treatment in patients with MALT lymphoma. We present a patient with MALT lymphoma involving stomach and lung which transformed to a pure plasma cell tumor after therapy with rituximab. This observation again supports the close association between the cell of origin of MALT lymphoma and plasma cells, suggesting that "plasmacytoma of the GI-tract" as anecdotally reported may in fact be a MALT lymphoma with extreme plasmacytic differentiation. In addition, our findings suggest that MALT lymphomas with plasmacytic differentiation might have a different 18F-FDG uptake as compared to classical MALT lymphoma.