Diagnosis of B-cell acute lymphoblastic leukemia (B-ALL) requires immunophenotypic evidence of B-lineage and absence of specific myeloid or T-lineage markers. Rare cases of otherwise typical B-ALL express myeloperoxidase (MPO) detectable by flow cytometry with an absence of other myeloid markers, but the clinical significance of this finding is not well studied.
A retrospective cohort analysis of flow cytometry and clinical data was performed to investigate the clinical outcome of this specific group of patients.
Twenty-nine cases of otherwise typical B-ALL that expressed MPO by flow cytometry (B-ALL-isoMPO) without expression of other myeloid markers were identified. The B-ALL-isoMPO group had a significantly increased incidence of relapse (univariate log rank P  = .0083; multivariate hazard ratio, 2.50; 95% confidence interval, 1.07-5.85; P  = .034) and significantly worse event-free survival by univariate analysis (log rank P  = .0066) compared with a reference group of patients with B-ALL from the same time period (n = 264).
To our knowledge, this is the first report to document the clinical outcomes in a group of pediatric patients with B-ALL that expresses MPO in the absence of other myeloid markers. This group had an increased rate of relapse and a worse event-free survival than the patients with B-ALL who did not express MPO.