SP31 | SUCCESSFUL FIRST-LINE TREATMENT OF ADVANCED-STAGE HEPATIC HISTIOCYTIC SARCOMA WITH TANDEM AUTOLOGOUS AND ALLOGENEIC STEM CELL TRANSPLANTATION: A CASE REPORT

Background: Histiocytic sarcoma (HS) is a rare, aggressive neoplasm originating from non-dendritic, non-Langerhans histiocytic cells of the monocyte-macrophage lineage. Localized HS is managed with surgery with or without radiotherapy, while advanced disease requires systemic chemotherapy and often results in poor outcomes. Due to the lack of standardized treatment, current approaches are extrapolated from aggressive lymphomas. Case presentation: We report the case of a 39-year-old male presenting with acute abdominal pain and laboratory signs of hepatic cytolysis and cholestasis. A contrast-enhanced total-body CT revealed a bulky hepatic lesion with abdominal lymphadenopathies and peripancreatic mass showing high FDG uptake on PET. Fine-needle aspiration of the hepatic lesion showed positive immunostaining for LCA, CD68, CD163, and CD4 while B-/T-cell, myeloid, epithelial, neuroendocrine markers were negative. NGS was negative for BRAF V600E, KRAS, NRAS, and MAP2K1 mutations. The patient underwent a pre-phase debulking therapy with two cycles of CHOP-14, achieving significant radiological response. Given the patient's fitness and the aggressive course of the disease, chemotherapy was escalated to ICE regimen (etoposide, carboplatin, ifosfamide) for four cycles. Following complete metabolic response on PET, the patient underwent ASCT after FEAM (fotemustine, etoposide, cytarabine, melphalan) conditioning, without any complication. Given the high risk of relapse and lack of targeted therapy, consolidative allo-SCT was considered the most suitable approach to achieve durable disease control. The procedure was performed under optimal conditions, given the patient's low transplant-related risk and the availability of an HLA-matched sibling donor. The conditioning regimen consisted of thiotepa and melphalan. GVHD prophylaxis included post-transplant cyclophosphamide, tacrolimus, and mycophenolate mofetil. The patient engrafted on day +15, with full donor chimerism on day +28. The post-transplant course was uneventful except for mild transient upper GI symptoms. Immunosuppression was discontinued on day +103. At 8 months post-transplant, the patient remains in complete remission without GVHD. Conclusion: Systemic HS remains a clinical challenge due to its poor prognosis. This case report may support the hypothesis that upfront high-dose chemotherapy combined with allogeneic cellular therapy could play a potential role in achieving durable complete remission.