Objective: This report aimed to present a rare clinical case of a male patient diagnosed with myeloid sarcoma with CBFB-MYH11 fusion of chromosome 16 inversion (inv(16)) in pleural fluid. Case presentation: Immunohistochemical analysis of the appendix, epiplon biopsy and immunophenotypic analysis of the patient's pleural fluid showed the presence of neoplastic cells with a phenotype suggestive of acute myeloid leukemia/myeloid sarcoma, and RT-PCR of the pleural fluid showed the presence of the CBFB-MYH11 fusion gene from inv(16). Discussion: Granulocytic myeloid sarcoma is a rare neoplastic disease that most commonly develops in soft tissue, bones, peritoneum, lymph nodes and gastrointestinal system. In these anatomical sites there is a proliferation of immature cells of the myeloid lineage. Diagnosis is performed by immunocytochemical, immunohistochemical and/or immunophenotypic analyses. The immunophenotype observed by the immunohistochemistry of appendix, epiplon biopsy and pleural fluid immunophenotyping were important for the diagnosis of myeloid sarcoma and are in agreement with what has been reported in the literature for the diagnosis of this disease. Conclusion: The joint analysis of biological samples by immunohistochemistry and immunophenotyping was essential for the correct diagnosis of the patient and, therefore, for the most appropriate therapeutic decision for this disease.
