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BACKGROUND: Most bone marrow biopsy specimens in multiple myeloma will show an abundance of typical neoplastic plasma cells with an eccentric basophilic nucleus and pale blue cytoplasm. Here, we describe morphologic and immunophenotypic features of an unusually rare variant of a neoplastic plasma cell discovered during bone marrow examination of a patient with multiple myeloma. We also provide further information on the diagnostic significance of such a cell when present in bone marrow aspirate.
CASE REPORT: We report the case of a 58-year-old man who presented with chronic left hip pain. Serum protein electrophoresis showed IgG Kappa and free Kappa monoclonal proteins. Skeletal survey showed a lytic lesion in the left femoral neck. Bone marrow biopsy showed a rare morphologic finding of an “atypical myeloma cell”, which is not commonly noted in most marrow specimens of multiple myeloma. This then prompted us to conduct the necessary immunostainings to guide us in arriving at the appropriate diagnosis. We report this rare pathologic entity together with its bone marrow biopsy images noted in our patient and the clinico-pathologic significance of such a finding.
CONCLUSIONS: Awareness of the existence of an atypical myeloma cell and it morphologic appearance while reviewing a bone marrow smear would enable the hemato-pathologist to consider multiple myeloma as a differential diagnosis and then conduct the necessary immunohistochemical stains to confirm this diagnosis. Differential diagnosis of this cell type from a crystal-storing histiocytosis is essential to arrive at the correct diagnosis since treatment ramifications are entirely different in each of these conditions.

Keywords: Histiocytes, Inclusion Bodies, Multiple Myeloma