A Rare Cohesive Cytomorphology in Primary Histiocytic Sarcoma of Bone Marrow with PD-L1 Expression: A case report and systemic literature review

Background: Histiocytic sarcoma (HS) is a rare hematologic malignancy. Primary bone marrow (BM) HS with cohesive clustered cytomorphology is rarely reported, posing diagnostic challenges. Herein, we report a case of atypical BM HS, which was confirmed by immunohistochemistry and had PD-L1 expression. Case presentation: A 66-year-old female presented with fever, retching and hypodynamia for 3 weeks. Peripheral blood examination showed aggressive pancytopenia accompanying with the positivity for Epstein-Barr virus (EBV) antibody; Computed tomography (CT) and abdominal ultrasound revealed hypersplenotrophy only, with no evidence of nodules or lymphadenectasis. BM aspirate had pleomorphic tumor cells (single diffuse/cohesive clusters), and BM smear hemophagocytes suggested lymphoma-associated hemophagocytic syndrome. Immunohistochemistry confirmed BM HS (CD4/CD68/CD163/PD-L1 positive; T-cell, B-cell and myeloid lineage markers negative). Conclusion: Cohesive clustered cytomorphologyis rare in primary BM HS. Its diagnosis requires combined immunophenotype and gene detection. In addition, PD-L1 may be a diagnostic marker and potential therapeutic target for this disease.