A 69-year-old man presented with cough and constitutional symptoms (white blood cell [WBC] count: 16.8 × 10⁹/L). A blood smear (Wright-Giemsa, 100× objective) showed mainly small lymphoid cells with nuclear indentations and eccentric nuclei (panel A). 5 days later, WBC count had risen to 138 × 10⁹/L (lactate dehydrogenase level: 1917 IU/mL to 12139 IU/mL). Cells became more pleomorphic, and developed features resembling acute monocytic leukemia (AML) with slightly larger forms, delicate chromatin, abundant agranular/basophilic cytoplasm, folded/lobated nuclei, and multiple nucleoli (panel B). Flow cytometry (peripheral blood) found an abnormal postthymic CD4⁺ T-cell population (62.5%) expressing CD15/CD52/CD30, loss of surface CD3/CD5/CD7. CD25/CD28/CD10 were negative. Lymph node biopsy indicated a diagnosis of anaplastic large cell lymphoma (ALCL): eosinophilic cytoplasm, nuclear indentations, and small nucleoli (panel C; hematoxylin-eosin 60× objective), some seen entirely filling lymph nodal sinuses (panel D; hematoxylin-eosin 40× objective); expressing CD3/CD2/CD5/CD4/CD30/CD15/GATA3 (panels E-K; immunoperoxidase 40× objective); BCL2/PAX5/MUM1⁺; EBV/granzyme-B/perforin/CD8/ALK1/EMA/TCR-β/FOXP3 negative; Ki-67 40%; normal karyotype; negative fluorescence in situ hybridization for ALK, DUSP22, and TP63 rearrangements; HTLV-1 serology negative. Bone marrow biopsy confirmed ALCL infiltration (>20%) with evidence of hemophagocytic lymphohistiocytosis (HLH). The patient died of sepsis and HLH following 2 cycles of chemotherapy.

This rare case demonstrates a significant morphological variability and evolution of leukemic ALCL, requiring differentiation from AML through an integrated diagnostic approach. Despite the intermediate prognosis of triple-negative (ALK⁻, TP63⁻, DUSP22⁻) ALCL, a leukemic presentation in itself appears to confer an unfavorable prognosis.