Abstract
This study describes an unusual case of multiple myeloma that progressed to anaplastic multiple myeloma in the pleural fluid. The Wright-stained cytospin of the pleural fluid showed a predominant population of mononuclear plasma cells with pleomorphic nuclei, characterized by both small and large nuclei, which is typical of anaplastic multiple myeloma. However, there were also more binuclear plasma cells with pleomorphic nuclei. Morphometric analysis showed that the mean nuclear length was 1.9-fold and 2.3-fold higher in the large nuclei compared to the small nuclei for the mononuclear plasma cells and binuclear plasma cells, respectively (p<0.001). The patient received B cell maturation antigen chimeric antigen receptor T cell (CAR-T) therapy for relapsed disease, with a significant reduction of the serum monoclonal paraprotein level at day 51 post-therapy. Pathologists should be aware that pleomorphic binuclear plasma cells can be part of the morphologic spectrum in anaplastic multiple myeloma.