Acute megakaryoblastic leukemia. Blast cell aggregates simulating metastatic tumor.

Acute megakaryoblastic leukemia is a rare leukemia that can present diagnostic problems. We describe two children who have this disease and had clumps of blast cells in their bone marrow, a finding usually attributed to metastatic tumor. The megakaryocytic origin of the cells was supported by their cytochemical staining pattern (positive alpha-naphthyl acetate esterase resistant to sodium fluoride inhibition and negative alpha-naphthyl butyrate esterase) and by the presence of factor VIII-related antigen. Ultrastructural studies of blast cells from one patient demonstrated platelet peroxidase. The mechanism of blast cell clump formation in these cases is unknown; nevertheless, awareness that this feature can occur in acute megakaryoblastic leukemia may avoid a misdiagnosis of metastatic solid tumor.