A Rare Case of Pediatric Chronic Myelogenous Leukemia Presenting with Severe Thrombocytosis without Leukocytosis.

Pediatric chronic myelogenous leukemia (CML) is uncommon. We report a pediatric patient with CML presenting with a normal white blood cell count and no circulating immature myeloid cells. The patient presented with extreme thrombocytosis (platelet count range: 2,175-3,064 x 109/L) noted incidentally. No splenomegaly was found. Examination of the bone marrow aspirate (BMA) revealed normal cellularity and normal myeloid: erythroid ratio with marked megakaryocytic hyperplasia. Molecular studies on the BMA detected both the major BCR/ABL1p210 fusion transcript (9,280 copies; p210/ABL1 ratio: 38.2%) and the minor p190 transcript (below limit of quantitation). The platelet count normalized within 2 weeks after treatment with the second-generation tyrosine kinase inhibitor dasatinib. Follow-up 3 months later revealed a 1.87 log reduction in p210 transcripts compared to diagnosis and no detectable p190 transcripts. This case highlights the need to include BCR/ABL1 fusion testing to accurately diagnose pediatric patients presenting with isolated thrombocytosis.