Acute megakaryoblastic leukemia (AMKL) is a rare subtype of acute myeloid leukemia (AML) that predominantly affects young children. The RBM15::MRTFA fusion gene is a recurrent genetic alteration in pediatric non-Down syndrome associated AMKL (n-DS-AMKL). Myeloid sarcoma (MS), an extramedullary solid tumor of immature myeloid cells, can be a presenting feature of AML and poses significant diagnostic challenges, often leading to initial misdiagnosis. We report the case of a 2-year-old boy who initially presented with a right frontal cranial mass. Imaging and initial histopathology were suggestive of Langerhans cell histiocytosis (LCH). Following surgical resection, the tumor recurred within four months. A subsequent biopsy of the intracranial lesion, prompted by the patient's deteriorating condition, revealed a malignant small round blue cell tumor. Immunohistochemistry was initially inconclusive, leading to a provisional diagnosis of a poorly differentiated vascular tumor. However, next-generation sequencing (NGS) analysis of the biopsy specimen identified the RBM15::MRTFA fusion gene, confirming the diagnosis of AMKL with MS. Bone marrow examination subsequently confirmed the leukemic involvement. The patient achieved complete remission after induction chemotherapy according to the SCCCG-AML 2020 protocol, followed by successful umbilical cord blood hematopoietic stem cell transplantation (HSCT). At the last follow-up, the patient remained in complete remission with no significant graft-versus-host disease (GVHD). This case highlights the critical role of comprehensive molecular profiling, particularly NGS, in the accurate diagnosis of AMKL presenting as myeloid sarcoma, especially when initial pathological findings are ambiguous. RBM15::MRTFA-positive AMKL can present with isolated extramedullary disease, and a high index of suspicion is necessary to avoid misdiagnosis. Early and precise genetic diagnosis is essential for guiding appropriate, potentially curative therapy, including HSCT, in pediatric patients.
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