Minimally differentiated acute myeloid leukemia: a morphologic, cytochemical and ultrastructural study

Seven of 368 cases of acute myeloid leukemia (AML) could not be subclassified by routine morphologic, cytochemical and immunologic analyses during the period January 1989 to December 1990. Further investigations including ultrastructural examination, anti-myeloperoxidase and myeloid specific antigen analysis were carried out in all these patients and they were classified as AML-MO, as per the FAB criteria. Morphologically these blasts resembled ALL-L2/AML-M1. Cytochemically they were negative for Sudan black, myeloperoxidase, periodic acid-Schiff, and non-specific esterase. On initial immunophenotypic analysis, they could not be classified into B, T or myeloid lineages. Further investigations revealed CD13 and CD33 positivity in 4 of 6 patients. Anti-myeloperoxidase was positive in 6 of 6 patients and ultrastructural examination revealed myeloperoxidase-positive blasts in 6 of 7 cases. Cytogenetic analysis done in one patient revealed 60% abnormal metaphases. Six of 7 cases were treated with aggressive chemotherapy. One patient achieved complete remission but relapsed after 6 months, whereas others were resistant to treatment. Hence we conclude that an aggressive investigative and therapeutic approach is required to identify and treat AML-MO.