The Efficiency of Allotransplant in a Case of Acute Biphenotypic Myeloid and B-Lymphoid Leukemia (MPAL Myelo/B NOS) That Presented Concurrently with a Mediastinal Granulocytic Sarcoma Co-Expressing Lymphoid Markers Complicated by Cardiac Tamponade

<b>Background and Clinical Significance</b>: Mixed-phenotype acute leukemia (MPAL) is a rare hematologic malignancy characterized by the co-expression of myeloid and lymphoid markers and is associated with poor prognosis. Myeloid sarcoma (MS), particularly in the mediastinum, is an uncommon extramedullary manifestation and is rarely reported in association with MPAL. <b>Case Presentation</b>: We report a rare case of mediastinal MS with biphenotypic features and pericardial extension occurring concurrently with MPAL, highlighting diagnostic challenges, therapeutic strategies, and long-term outcomes. We describe the clinical course, diagnostic workup, treatment, and follow-up of a 21-year-old woman who presented with cardiac tamponade secondary to a mediastinal mass. Histopathology and immunophenotyping established the diagnosis of mediastinal MS associated with MPAL (B/myeloid, NOS). Management included surgical cytoreduction, intensive induction chemotherapy, and consolidation with allogeneic hematopoietic stem cell transplantation (allo-HSCT) from an unrelated donor. Fertility preservation with oocyte retrieval, in vitro fertilization (IVF), and embryo cryopreservation was performed prior to conditioning. A focused literature review of MPAL cases with extramedullary involvement was conducted. The patient achieved complete remission following induction therapy and underwent allo-HSCT. Despite the historically poor prognosis of mediastinal MS and MPAL, she remains in sustained complete remission 13 years after diagnosis. A literature review identified only eight reported cases of MPAL with extramedullary disease, with mediastinal involvement described in a single case and allo-HSCT performed in only two patients. <b>Conclusions</b>: This case illustrates a rare presentation of MPAL with mediastinal myeloid sarcoma and cardiac tamponade, demonstrating that aggressive multimodal therapy including allo-HSCT may achieve durable remission even in high-risk presentations. Early multidisciplinary management and consideration of fertility preservation are essential in young patients.