Incidence and immunophenotype of abnormal lymphoblast populations at diagnosis of chronic myeloid leukaemia in chronic phase

Chronic myeloid leukaemia most commonly presents in chronic phase (CML-CP) and it is characterised by granulocytic proliferation. Many patients have an excellent response to tyrosine kinase inhibitor therapy; however, a small proportion will develop lymphoid or myeloid blast crisis, with inferior clinical outcomes. Detection of lymphoblasts at diagnosis of CML-CP has been reported in small case series with conflicting results on the risk of subsequent blast crisis. The aim of this study was to identify the incidence and immunophenotype of abnormal lymphoblast populations in CML-CP. Retrospective review of bone marrow flow cytometry results of consecutive patients with newly diagnosed CML-CP between June 2012 and February 2021 was performed. Lymphoblasts, myeloblasts, haematogones, and mature lymphocytes were evaluated. Fifty-nine patients had bone marrow flow cytometry results available for review. Abnormal lymphoblast populations were detected in four patients (7%) comprising 0.05–0.19% of bone marrow events. The immunophenotype was similar but distinct from haematogones. The most common distinguishing features of the abnormal lymphoblast populations were abnormally bright expression of CD19 or CD10, weak CD38 or aberrant CD20 expression on CD34 + cells. The clinical case of one of the patients with abnormal lymphoblasts detected at diagnosis who went on to subsequent blast crisis is discussed. Abnormal lymphoblasts can be identified in CML-CP and may be under-recognised. Their detection requires careful analysis in order to distinguish them from normal precursors. The clinical significance of such populations requires further study.