Mott cells in CD20‐positive myeloma

Abstract

A 63-year-old woman presented with a 2-month history of left thigh pain, no ‘B’ symptoms, no significant past medical illness and normal clinical examination. Her full blood count was normal. The erythrocyte sedimentation rate was elevated at 115 mm/h, total globulins were elevated but the rest of the serum biochemistry was normal. Serum immunoglobulin electrophoresis with immunofixation showed an IgGλ paraprotein at a concentration of 22 g/l and elevated IgM at 10·4 g/l. Urinary Bence–Jones protein was absent and the serum free light chain ratio was normal. A skeletal survey was normal. A bone marrow aspirate showed a lymphoplasmacytic infiltrate of 30% and cells with large intracytoplasmic inclusions (top right) often in doublets or triplets with the nuclei pushed to the rim of the cell. A bone marrow trephine biopsy showed extensive infiltration by lymphoplasmacytic cells and huge plasma cells (top left). Plasma cells with large Russell body inclusions occupied both paratrabecular and intertrabecular areas and stained positively for CD20, nuclear Cyclin D1 and lambda light chain (bottom right). Flow cytometry initially failed to identify light chain restricted plasma cells. It was recognised that the large sized plasma cells were outside the conventional lymphoid/plasma cell gate. Repeat ungated flow cytometric analysis demonstrated CD45+ cytoplasmic kappa+ = 11%; CD45+ cytoplasmic lambda+ = 17%; CD19+ cytoplasmic kappa+ = 12%; CD19+ cytoplasmic lambda+ = 14% and a reversed kappa: lambda ratio of 11:17. Fluorescence in situ hybridisation on separated plasma cells (CD138+ve) was carried out using the IGH/CCND1 dual fusion dual colour probe (Vysis, Berkshire, UK). Of 100 cells analysed, 78 showed a pattern of one fusion signal, which indicated the presence of the t(11;14) translocation (bottom left). The combined clinicopathologic features confirmed the diagnosis of myeloma. The patient remains asymptomatic and no therapeutic options have yet been offered. This case demonstrates an infrequent but impressive morphological appearance of plasma cells with huge intracytoplasmic immunoglobulin inclusions (Mott cells). The presence of t(11;14) is most commonly associated with mantle cell lymphoma but is found in up to 20% of patients with myeloma, using interphase cytogenetics. Lymphoplasmacytoid appearances, high CD20 expression and t(11;14) has been described as a sub-entity in myeloma. This case demonstrates the importance of always reviewing the flow cytometry data in conjunction with morphology.