Mixed-phenotypic acute leukemia: cytochemically myeloid and phenotypically early T-cell precursor acute lymphoblastic leukemia

TO THE EDITOR: Early T-cell precursor acute lymphoblastic leukemia (ETP-ALL) is characterized by the specific immunophenotype of CD1a, CD8, and CD5 with stem cell or myeloid marker expression. This leukemia presents a particular flow cytometric diagnostic challenge, as it frequently exhibits sufficient myeloid-lineage differentiation to be classified as mixed-phenotype acute leukemia (MPAL). The 2008 World Health Organization classification has established new strict criteria for the diagnosis of MPAL; however, the exact MPO positivity cut-off percentage required for establishing a myeloid lineage was not mentioned. Herein, we report a case of ETP-ALL with trisomy 4 and t(6;14)(q27:q22) in a 25-year-old man, the final diagnosis of which was MPAL (T/Myeloid) due to the morphologic presence of Auer rods and myeloperoxidase staining, despite exhibiting a flow cytometric ETP-ALL phenotype. To the best of our knowledge, this is the first report on a case of ETP-ALL with Auer rods.