Acute megakaryocytic leukemia with myeloid/monocytic differentiation.

Acute megakaryocytic leukemia is a rare form of acute nonlymphocytic leukemia that occurs with increased frequency in patients with Down's syndrome. Herein, we report a child with Down's syndrome who presented with a large retroperitoneal mass due to acute megakaryocytic leukemia. Immunohistochemical stains of the tumor cells also demonstrated evidence of myeloid/monocytic differentiation, with positivity for alpha-1-antitrypsin, alpha-1-chymotrypsin, and lysozyme. The significance of this phenotypic heterogeneity is unclear and awaits further studies of similar cases.