Extramedullary Involvement in T/Myeloid Mixed Phenotype Acute Leukemia With BCR::ABL1 Fusion in a Hispanic Female Patient: A Case Report.

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 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 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.