A rare case of pediatric T-cell acute lymphoblastic leukemia with myeloid mimicry.

Acute lymphoblastic leukemia (ALL) with granular blasts was historically referred to as "granular ALL", but this is not recognized as a distinct entity in the current WHO classification. This rare morphological feature is predominantly associated with a B-cell immunophenotype, while T-ALL with prominent cytoplasmic granules is extremely rare, with limited available genetic, molecular, and prognostic data. Here, we report a unique case of ALL in a 9-year-old girl. Initial blood tests revealed leukocytosis, and both peripheral blood and bone marrow morphology showed a high number of blasts with granular cytoplasm and bilobed nuclei. Immunophenotyping confirmed T-cell origin. The patient tested positive for KMT2A::AFDN, with WT1 overexpression, and NRAS and EZH2 mutations were detected by next-generation sequencing (NGS). Remarkably, the patient achieved complete remission after treatment and has shown no signs of relapse for nearly four years. Her favorable response to conventional therapy underscores the importance of molecular phenotyping in the treatment of this disease. The continued use of NGS to gather relevant molecular data is crucial for further understanding the molecular phenotype and prognosis of such atypical ALL cases.