Extramedullary myeloid cell tumors arising in the setting of chronic myelomonocytic leukemia. A report of two cases.

We report two cases of extramedullary myeloid cell tumor that arose in patients with chronic myelomonocytic leukemia. In both cases, the tumors were difficult to recognize histologically because the neoplasms lacked cytological evidence of granulocyte maturation, such as cytoplasmic granulation or eosinophilic myelocytes, and the Leder stains for chloroacetate esterase were negative. Immunohistochemical studies were necessary to establish the correct diagnosis. The neoplastic cells in both tumors expressed myeloperoxidase, lysozyme, and CD43 and were negative for B-cell, T-cell, and other nonhematopoietic antigens tested. We report these cases to emphasize that extramedullary myeloid cell tumors may rarely precede transformation to acute myeloid leukemia in patients with chronic myelomonocytic leukemia. Extramedullary myeloid cell tumors of monocytic lineage may be difficult to recognize in routine and Leder-stained sections, and immunohistochemical studies may be essential for establishing the diagnosis.