Abstract
Mixed phenotype acute leukemia (MPAL) is a rare subtype of acute leukemia characterized by the expression of markers from more than one lineage. The T/myeloid subtype, especially with extramedullary involvement and BCR::ABL1 fusion, is exceptionally rare and diagnostically challenging. We report a case of a 61-year-old Hispanic female patient presenting with generalized lymphadenopathy. Excisional biopsy of a left occipital lymph node showed diffuse effacement by a monomorphic population of blasts. Immunohistochemistry revealed co-expression of myeloid (partial MPO, lysozyme) and T-lineage markers (CD3, CD4, CD5, CD7). Bone marrow biopsy confirmed MPAL with extensive infiltration by CD3+, CD5+, and CD2+ cells; approximately 20-30% co-expressed CD34, CD117, and TdT. Flow cytometry supported a diagnosis of T/Myeloid MPAL. Fluorescence in situ hybridization (FISH) analysis identified BCR::ABL1 rearrangement. This case highlights the importance of integrating morphology, immunophenotyping, and molecular testing to diagnose MPAL and underscores the need for clinical awareness of its varied presentations, including extramedullary disease.