Adult mixed phenotype acute leukemia (MPAL): B/myeloid MPALisoMPO is distinct from other MPAL subtypes

Abstract Introduction Myeloperoxidase (MPO) is considered a specific marker of myeloid/non‐monocytic lineage in the diagnosis of mixed phenotype acute leukemia (MPAL). However, the clinical significance of isolated dim MPO expression in otherwise typical B lymphoblastic leukemia (B‐ALL; referred to as B/myeloid MPAL isoMPO ) in adult patients is unknown. Methods We compared flow cytometric immunophenotype and clinicopathological findings among cases of B/myeloid MPAL isoMPO ( n = 13), other MPAL subtypes ( n = 10, B/myeloid and T/myeloid MPAL), B‐ALL ( n = 64), and acute myeloid leukemia (AML, n = 58), using the 2016 WHO classification. For MPAL cases, MPO was reported as the percent of MPO positive blasts and its intensity (dim or moderate/strong). The pattern of heterogenous antigen expression (inversely coordinated expression between myeloid and lymphoid markers and cell size) was assessed. Results Cases of B/myeloid MPAL isoMPO showed a fairly homogenous single B‐lineage blast population with dim MPO expression whereas cases of other MPAL subtypes displayed heterogeneous antigen expression and moderate/strong MPO expression. The percent of MPO positive blasts in these two groups was similar. Expressions of CD15, CD117, and monocytic markers were more common in other MPAL than in B/myeloid MPAL isoMPO . B/myeloid MPAL isoMPO patients had similar overall and leukemia free survivals as B‐ALL patients and better than other MPAL patients. Conclusion This is the first study to investigate the clinical significance of adult B/myeloid MPAL isoMPO using the 2016 WHO classification. Our results suggest that B/myeloid MPAL isoMPO clinically behaves more similarly to B‐ALL than to other MPAL subtypes, supporting the 2016 WHO classification to segregate this entity from other MPAL subtypes.