Masked by eosinophils: a cryptic presentation of pediatric B-ALL with IGH rearrangement.

Hypereosinophilia is uncommon in the pediatric population and may be associated with either primary or secondary conditions. In a very small proportion of cases (less than 1%), it forms part of the initial presentation of acute lymphoblastic leukemia (ALL). More frequently, hypereosinophilia appears before overt leukemia symptoms develop, which can delay diagnosis and place affected children at increased risk due to the lack of early intervention. We recently reviewed a case of 6½-year-old female who presented with hypereosinophilia and no circulating blasts. Bone marrow aspirate showed significant prominence of eosinophils and their precursors with ~ 2% blasts on morphology. Bone marrow biopsy was received for review and showed a significant burden of immature cells/blasts which on immunohistochemistry was revealed as B-acute lymphoblastic leukemia (B-ALL). Cytogenetics by fluorescence in situ hybridization (FISH) showed IGH rearrangement while the routine karyotyping as well as next-generation sequencing (NGS) were both negative for any genetic aberrancy. This case highlights the importance of a thorough marrow evaluation in pediatric eosinophilia and discusses the challenges in identifying cryptic rearrangements that are not evident on standard cytogenetic and molecular panels.