A rare cohesive cytomorphology in primary histiocytic sarcoma of bone marrow with PD-L1 expression: a case report and systematic literature review

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. 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 splenomegaly only, with no evidence of nodules or lymphadenopathy. BM aspiration and biopsy revealed diffuse infiltration by large pleomorphic malignant cells forming numerous cohesive clusters. Immunohistochemistry finally confirmed BM HS (CD4/CD68/CD163/PD-L1 positive; T-cell, B-cell and myeloid lineage markers negative). Cohesive clustered cytomorphology is 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.