Acute megakaryoblastic leukemia mimicking small round cell tumor with novel t(1;5)(q21;p13).

Acute megakaryoblastic leukemia is a relatively rare form of acute leukemia that has heterogeneous blast morphology and karyotypic abnormalities. An 8-month-old boy with a retroperitoneal mass was diagnosed as having acute megakaryoblastic leukemia that initially presented as small round cell tumor of childhood. Bone marrow aspiration showed syncytial groups of atypical medium sized cells with scant cytoplasm and fine nuclear chromatin.