*6.10.1 Morphology and immunophenotype.*

Multiple myeloma is characterized by infiltration of marrow by plasma cells in the interstitium, nodules or in diffuse sheets that completely replace normal elements (**Figure 9**). Plasma cells can be relatively normal-appearing plasma cells, plasmablasts or binucleated and multinucleated cells. Other cytologic

**Figure 9.** *Bone marrow biopsy showing sheets of plasma cells replacing normal hematopoietic elements.*

variants include flame cells with fiery red cytoplasm, Mott cells with multiple grapelike cytoplasmic droplets, and cells containing a variety of other inclusions—Russell bodies (cytoplasmic) or Dutcher bodies (nuclear), including fibrils, crystalline rods, and globules. Peripheral blood smears shows rouleaux formation. Rarely, tumor cells enters the peripheral blood, giving rise to plasma cell leukemia. Plasma cell tumors are positive for CD 38, CD138, and often express CD56, a feature that can be helpful in identifying small populations of neoplastic cells.
