Acute leukemia of megakaryocyte lineage with tumor formation. An autopsy case of patient with Down's syndrome.

In this report we present the autopsy findings of acute megakaryoblastic leukemia with tumor formation in a 2-year-old female infant with Down's syndrome. Chromosomal analysis of blast cells revealed constitutional anomaly of trisomy 21 and two other related types of abnormal clones. Flow cytometric examination revealed blast cells expressing Ia-like or HLA-DR antigens. Postmortem examination showed extensive infiltration of leukemic cells in most of the examined organs, including the bone marrow with myelofibrosis. Tumor masses in the maxillary, frontal and femoral bones and the atria of the heart had undergone massive infiltration of atypical blast cells with an increase in the reticulin network. The final diagnosis was confirmed by ultrastructural cytochemistry of the platelet peroxidase reaction as well as by immunological staining utilizing anti-platelet glycoprotein IIb/IIIa, antiplatelet factor 4 and anti-beta-thromboglobulin antibodies for the blast cells. It seems likely that platelet-derived growth factor, secondary to an increase in the reticulin network, plays a major role in myelofibrosis of acute megakaryoblastic leukemia with tumor formation.