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Neoplastic plasma cells
may show a variety of morphologic features, including mature,
intermediate, immature, and plasmablatic features. Some myeloma
cells may even be lymphoid-appearing. Therefore, it is probably
worthwhile to perform CD138 immunostain if no apparent plasma
cells are noted in the marrow
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Also, solitary
plasmacytoma may present at the extramarrow sites without
typical clinical presentations or marrow involvement;
occasionally, the tumor cells may not be plasma cell-appearing at
all, which can resemble poorly differentiated carcinoma, sarcoma,
melanoma or lymphoma. Therefore, plasmacytoma should be considered
as one of the differential diagnoses for a wired tumor of unknown
origin
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