A peculiar case of acute myeloid leukemia mimicking plasmacytoid dendritic precursor cell leukemia.

Differential diagnosis between plasmacytoid dendritic precursor cell leukemia (pDC leukemia) and acute myeloid leukemia (AML) with monocytic differentiation is difficult due to shared clinicopathological features ; however, such diagnosis is critical because the two leukemias are treated differently. Here we report a peculiar case of AML mimicking pDC leukemia. A 22-year-old man presented with leukocytopenia and bone marrow involvement of atypical plasmacytoid cells with a prominent nucleolus. In spite of positive cytochemical staining for NaF-sensitive naphthyl butyrate esterase, this case was diagnosed as pDC leukemia because the abnormal cells were positive for CD4, CD56, and CD123, and negative for myeloperoxidase and lysozyme. The patient achieved complete remission after 4 courses of combination chemotherapy, but relapsed four months later with leukemic manifestation and skin involvement. The morphology of the leukemia cells became myelomonoblastic, and some were immunohistochemically positive for lysozyme, suggesting AML. Although the patient received allogenic stem cell transplantation twice, he died of progressive disease. This case demonstrates the importance of cytochemical staining for naphthyl butyrate esterase in differential diagnosis between AML and pDC leukemia coexpressing CD4, CD56, and CD123.