Extramedullary (skin) presentation of acute monocytic leukemia resembling cutaneous lymphoma: morphological and immunological features.

Acute monocytic leukemia has been noted to exhibit a predilection for extramedullary involvement (gums, skin, and lymph nodes) at presentation. More unusual is the occurrence in an extramedullary site in the absence of bone marrow involvement. A case is reported with initial presentation in skin preceding a subsequent evolution to a leukemic phase by one year. The skin tumor was initially diagnosed and treated as a lymphoma. A second skin tumor, biopsied one year later was immunophenotyped as a T cell lymphoma using a screening panel of antisera (OKT4 positive, OKMI negative). Shortly thereafter a monocytic leukemia (M5) was discovered. Using a larger panel of antisera and enzyme markers on the second skin biopsy confirmed the monocytic rather than lymphocytic nature of the skin tumor. This case illustrates the importance of using an expanded panel of monoclonal antisera in certain hematopoietic tumors.