Identification of an NFIA::CBFA2T3 fusion in cerebrospinal fluid confirms the diagnosis of pediatric CNS myeloid sarcoma with erythroid differentiation: a case report and literature review.

Myeloid sarcoma with erythroid differentiation represents a mass-forming presentation of acute erythroid leukemia. This entity is exceptionally rare in the pediatric population, with only sporadic reports of de novo cases predominantly involving the central nervous system or orbit. Diagnosing myeloid sarcoma with erythroid differentiation poses significant clinical and pathological challenges, particularly in cases without bone marrow involvement. Here, we report the case of a 1-year-old boy with myeloid sarcoma with erythroid differentiation exhibiting diffuse parenchymal brain infiltration without mass formation. Due to the patient's critical condition, a tissue biopsy was unfeasible. However, cerebrospinal fluid (CSF) flow cytometry revealed a significant population of immature erythroid cells, and RNA sequencing identified an NFIA::CBFA2T3 fusion-a genetic alteration previously reported in multiple myeloid sarcoma with erythroid differentiation cases. Notably, molecular testing confirmed that the patient was negative for both TP53 mutation and chromosome 17 loss. Given the diagnostic complexity of this tumor, both flow cytometry and RNA sequencing played pivotal roles in establishing the definitive diagnosis.