Retroperitoneal and Pelvic Myeloid Sarcoma in a Patient With JAK2-Positive Polycythemia Vera: A Case Report.

Myeloid sarcoma (MS) is a rare extramedullary tumor composed of immature myeloid cells that may occur in association with acute myeloid leukemia or other myeloid neoplasms. We report a 75-year-old woman with JAK2-positive polycythemia vera who presented with abdominal pain, distension, and anorexia. Imaging revealed a large retroperitoneal and pelvic mass causing hydronephrosis. Biopsy demonstrated immature mononuclear cells positive for CD33, CD43, CD117, CD68, and CD163, consistent with MS with monocytic differentiation. Other malignancies were excluded, and flow cytometry was non-diagnostic. This case highlights the diagnostic challenge of MS and underscores the importance of integrating imaging, histopathology, and immunophenotyping for accurate diagnosis. Early recognition is critical, as untreated MS can progress to acute leukemia. Clinicians should consider MS in patients with myeloproliferative neoplasms presenting with atypical masses to guide timely therapy and improve outcomes.