A Challenging Case of A Myeloid Sarcoma Misdiagnosed as High Grade B-Cell Lymphoma. (28th October 2020) - Herrmann, A
Abstract: Introduction/Objective: A 46-year-old female presented with severe back pain associated with progressive bilateral lower extremity weakness and paresthesia, urinary retention, and constipation. Computed tomography revealed a retroperitoneal mass encasing the right psoas muscle, obstructing the right kidney, and extending to the thoracolumbar region resulting in severe spinal compression. An epidural tumor resection was subsequently performed at an outside hospital. Methods: Histological sections showed sheets of blastoid neoplastic cells with intermediate to large nuclei, irregular membranes, fine chromatin, and prominent nucleoli. Immunohistochemical stains showed that these cells were positive for CD43, CD79a (weak, focal), BCL2, C-MYC, and PAX5 (weak, focal) and negative for CD10, CD20, CD30, ALK1, BCL6, MUM1, and Tdt. The Ki-67 proliferation index was 75-80%. With this immunophenotype, this patient was diagnosed with a high grade B-cell lymphoma and transferred to our institution for further work-up. On review of the slides, further immunohistochemical testing was requested which revealed positivity for CD117 and myeloperoxidase (MPO). Results: The overall morphological and immunophenotypical features are most compatible with myeloid sarcoma (MS) with aberrant expression of B-cell markers and this patient's diagnosis was amended. Interestingly, the patient's bone marrow examination only showed 2% myeloblasts with left shifted granulocytosis and concurrent fluorescence in situ hybridization (FISH) studies were negative. Conclusion: A literature review showed that 40-50% of MS are misdiagnosed as lymphoma. MS can frequently stain with B-cell or T-cell markers, as seen in this case, which makes it challenging for an accurate diagnosis and sub- classification. In addition, our case is interesting in that there was only extramedullary presentation without bone marrow involvement. Typically, MS develops after the diagnosis of acute myeloid leukemia (AML) with an incidence of 3–5% after AML. It can also manifest de novo in healthy patients, who then go on to develop AML months to years later. Therefore, this patient will require close follow-up.