Essential thrombocythaemia with aggressive megakaryocytosis after myelofibrotic transformation.

BACKGROUND Among myeloproliferative neoplasms, it is often difficult to distinguish essential thrombocythaemia (ET) from prefibrotic-stage primary myelofibrosis (PMF) with thrombocytosis given their overlapping clinicopathological phenotypes. CASE PRESENTATION We encountered a 45-year-old male who was initially diagnosed with ET and eventually became transformed to secondary myelofibrosis 20 years later. Two distinct types of aberrant megakaryocytes were observed at diagnosis: one type characteristic of ET and the other type characteristic of PMF. With a proliferation in the bone marrow, aberrant megakaryocytes were infiltrated into the extramedullary organs and were even present in the thrombus were observed at autopsy. As a result of next-generation sequencing, the significant increase of variant allele frequency (VAF) of JAK2 V617F and U2AF1 S34Y mutations was observed in the bone marrow cells at the final stage. CONCLUSIONS This patient could be recognized as an atypical case of aggressive megakaryocytosis transformed from ET.