Durable Response in Histiocytic Sarcoma After Allogeneic Stem Cell Transplantation: A Case Report

: Background and Clinical Significance: Histiocytic sarcoma (HS) is a rare and aggressive form of malignant histiocytosis, often associated with poor prognosis. The diagnosis and management of HS are challenging due to the complexity of its pathogenesis, molecular profile, and the unclear cellular origin of histiocytic neoplasms, compounded by the limited literature on treatment strategies. Case Presentation: We report the case of a young patient with HS localized to the lymph nodes, spleen, and liver, who also presented with hemophagocytic lymphohistiocytosis (HLH) documented on bone marrow biopsy. Initial treatment with CHOEP-21 and ICE-21 chemotherapy resulted in only a partial metabolic response, as evidenced by a Fluorodeoxyglucose-Positron Emission Tomography (FDG-PET)/CT scan. Given the aggressive nature of the disease and the presence of HLH, an allogeneic hematopoietic stem cell transplantation (HSCT) from a matched unrelated donor was performed as consolidation therapy, leading to a progressive complete response without significant toxicity. A suspected relapse at 18 months post-transplant was excluded following a mediastinal lymph node biopsy, which revealed a benign intravascular papillary endothelial hyperplasia (IPEH). Over five years post-diagnosis and more than four years after transplantation, the patient remains in complete remission with full functional recovery. Conclusions: This case highlights the diagnostic and molecular challenges of HS and demonstrates the curative potential of early allogeneic HSCT, even when only partial remission is initially achieved.