Hidden in the mediastinum: a case of extramedullary hematopoiesis unveiled through thoracoscopy.

Extramedullary hematopoiesis (EMH) is a rare condition characterized by proliferation of hematopoietic stem cells outside the bone marrow, usually as a compensatory response to hematological disease. Although EMH primarily occurs in the liver and spleen, it can manifest in atypical locations, such as the mediastinum. We herein describe an asymptomatic 66-year-old man with incidentally discovered posterior mediastinal EMH. A 28- × 32-mm mass was detected during a routine examination. Laboratory findings were within normal limits. Computed tomography revealed a well-defined enhancing mass with a density of 60 Hounsfield units, suggestive of a neurogenic tumor. Surgical resection confirmed EMH, characterized by megakaryocytes and hematopoietic precursors. The patient recovered smoothly and was discharged 5 days postoperatively. Accurate preoperative diagnosis of EMH is challenging, as illustrated by this case. Although typically associated with anemia or hematological abnormalities, EMH can present without such signs. Surgical resection and histopathological examination are essential for diagnosis. This case emphasizes the diagnostic complexity of posterior mediastinal EMH, even in patients without overt hematological disorders. Posterior mediastinal EMH is exceedingly rare and diagnostically demanding. A high index of suspicion and histological tissue analysis are crucial for optimal management. Video-assisted thoracoscopic surgery enables both diagnosis and treatment through mass excision.