Anaplastic myeloma with massive extramedullary involvement. Report of two cases.

Two patients developed anaplastic myeloma associated with prominent extramedullary disease one and four years after the initial diagnosis of multiple myeloma. Most tumor cells in the extramedullary sites were primitive-appearing and anaplastic, although plasmacytoid differentiation was evident. One patient had concurrent bone marrow involvement by cytologically similar cells, while the other patient was in bone marrow remission when he developed clinically evident soft tissue infiltration. Immunoperoxidase staining showed similar cytoplasmic immunoglobulin in both the mature-appearing and anaplastic cells in both patients. Electron microscopy in both cases showed a morphologic spectrum between the mature-appearing and anaplastic cells. DNA flow cytometric studies following the onset of extramedullary disease revealed a tumor cell population with a high degree hyperdiploid abnormality and unusually high proliferative activity in both cases, consistent with the morphologic anaplasia. Anaplastic myeloma appears to represent a distinct, aggressive variant of multiple myeloma which may result from a transformation of well differentiated myeloma cells to poorly differentiated immunoblast-like cells.