Acute Monocytic Leukemia With Histiocyte-Like Morphology and Trisomy 8: A Rare Diagnostic Challenge.

Acute monocytic leukemia (AML-M5) is a subtype of acute myeloid leukemia characterized by the proliferation of monoblasts and promonocytes showing monocytic differentiation. Although typical cytomorphological features are usually recognizable, unusual presentations may create diagnostic difficulties. We report a rare case of AML presenting with prominent histiocyte-like cells in the bone marrow associated with trisomy 8. Bone marrow examination revealed hypercellularity with two predominant cell populations consisting of monocytoid blasts and large histiocyte-like cells exhibiting abundant foamy cytoplasm and occasional hemophagocytosis. Cytochemical staining for myeloperoxidase was negative. Flow cytometry demonstrated an abnormal population expressing CD33, CD15, and CD4 with weak CD45 expression, while lymphoid markers were negative. Cytogenetic analysis identified trisomy 8 in the majority of metaphases. The patient received standard induction chemotherapy followed by consolidation therapy according to conventional AML protocols. This case highlights the importance of integrating cytomorphology, immunophenotyping, and cytogenetic findings to establish the correct diagnosis when atypical histiocytic-like features are present.