Extramedullary T-Lymphoblastic Blast Crisis in Chronic Myeloid Leukaemia: A Unique Cytologic Diagnosis Confirmed by Flow Cytometry.

BACKGROUND: Extramedullary blast proliferation (EBP) occurs in approximately 4%-16% of patients with chronic myeloid leukaemia (CML) in blast phase and may precede overt marrow blast transformation. Although myeloid lineage predominates, lymphoid blast crises account for approximately 20%-30% of cases, with T-lineage differentiation representing only a small minority. Accurate recognition is important because EBP defines blast-phase disease irrespective of the bone marrow blast percentage. CASE PRESENTATION: A 40-year-old man presented with generalized lymphadenopathy, hepatosplenomegaly, anaemia, and marked leukocytosis. Bone marrow examination demonstrated CML in chronic phase, while fluorescence in situ hybridization confirmed BCR::ABL1 fusion. Fine needle aspiration cytology (FNAC) from a cervical lymph node revealed a monomorphic population of intermediate-sized blasts with a high nuclear-cytoplasmic ratio and scant agranular cytoplasm. Flow cytometric immunophenotyping performed on the aspirate demonstrated expression of cytoplasmic CD3, CD5, CD7, CD34, CD38, CD33, and CD117, with absence of myeloperoxidase, CD4, and CD8, establishing a diagnosis of extramedullary T-lymphoblastic blast crisis. CONCLUSIONS: This case highlights the value of FNAC combined with flow cytometry in diagnosing extramedullary blast proliferation. Awareness of this rare manifestation is essential because it may represent the first evidence of blast-phase transformation despite chronic-phase morphology on bone marrow examination.