| Summary: | Chronic lymphocytic leukemia (CLL) is known to be associated rarely with myeloid malignancies such as acute myelogenous leukemia. In this article, we report an extremely rare occurrence of acute promyelocytic leukemia in a patient with CLL. A 71-year-old man first presented to our clinic with a diagnosis of CLL and worsening motor neuropathy symptoms. It was suspected that his CLL might be contributing to the neuropathy as a paraneoplastic syndrome, and he was treated with rituximab monotherapy in weekly doses for the 1st month and monthly treatments thereafter. By the end of his sixth monthly course of rituximab, the patient noted significant improvement in neuropathy symptoms but reported experiencing a new-onset worsening fatigue. He had new-onset cytopenias (white blood cells 1.6k/µL, hemoglobin 11.7g/dL, and platelet count 77k/µL). A bone marrow examination was performed; it showed a high percentage of progranulocytes (21%), which stained positive for myeloperoxidase (MPO) and demonstrated a fine granular pattern on the promyelocytic leukemia (PML) oncogenic domain immunofluorescence test. The diagnosis of acute promyelocytic leukemia was confirmed by fluorescence in situ hybridization, which showed a PML/RARα rearrangement in 46% of interphases. Flow cytometry was consistent with immunophenotype of acute promyelocytic leukemia and minimal residual CLL (0.07%). The patient was started promptly on all-trans-retinoic acid and arsenic trioxide induction regimen. Molecular remission was achieved after the first consolidation cycle. The patient is currently past his fourth consolidation cycle of all-trans-retinoic acid/arsenic trioxide and continues to be in complete remission. Our case illustrates that it is important for the physicians to be aware of coexistent hematologic and solid tumor malignancies in CLL, and maintain a low threshold for diagnostic testing based on grounds of low clinical suspicion. Keywords: Acute promyelocytic leukemia, Acute myeloid leukemia, Chronic lymphocytic leukemia, Rituximab
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