Lineage Assignment in Acute Leukemia: A Challenging Case in a Pediatric Patient

We report a case of a 2-year-old girl who was diagnosed with natural killer cell acute lymphoblastic leukemia and treated with an acute lymphoblastic leukemia chemotherapy regimen. Two months posttherapy, the disease relapsed with a myeloid immunophenotype. Complete response was then achieved with acute myeloid leukemia therapy followed by unrelated donor umbilical cord allogenic stem cell transplant. Retrospectively, reanalysis of the diagnostic specimen showed minimal myeloperoxidase expression that was called negative by conventional single parameter linear gating but better appreciated on histogram overlays. This case illustrates that even low levels of myeloperoxidase expression should be considered significant in lineage assignment in acute leukemia.