167 Malignant Pericardial Effusion as a Rare Extramedullary Manifestation of Relapsed Hairy Cell Leukemia variant

Introduction/Objective Hairy cell leukemia variant (HCLv), now classified as splenic B-cell lymphoma/leukemia with prominent nucleoli (SBLPN), is a rare mature B-cell neoplasm accounting for approximately 0.4% of chronic lymphoid malignancies and about 10% of hairy cell leukemia cases. It is typically characterized by a CD25-negative/CD123-negative immunophenotype and BRAF wild-type status. Extramedullary involvement is rare, and reported extranodal, skeletal, soft-tissue, and pericardial cases are limited to isolated case reports. Methods/Case Report An 80-year-old male with a history of HCLv (treated with cladribine and rituximab in 2020) presented in 2025 with progressive exertional dyspnea. Imaging confirmed a large circumferential pericardial effusion with early tamponade physiology. The patient underwent CT-guided pericardiocentesis for cytologic and flow cytometric evaluation. Cytomorphology showed numerous atypical mononuclear cells with abundant cytoplasm, occasional fine hair-like projections, and relatively inconspicuous nucleoli. Flow cytometry identified a 91% monoclonal, lambda-restricted B-cell population with bright CD20, CD22, CD103, and CD11c expression. Notably, the cells displayed uncharacteristic strong and uniform CD25 positivity, indicating an immunophenotypic shift from the CD25-negative primary diagnosis. By IHC Annexin A1 staining was present only in a subset of cells and repeat BRAF V600E testing remained negative. Results NA Conclusion This case presents an exceptionally rare pericardial manifestation of relapsed Splenic B cell lymphoma (Hairy Cell Leukemia variant). It emphasizes the potential for immunophenotypic evolution in HCL-v mimicking classic HCL after therapy and highlights the importance of integrating morphologic and molecular findings for accurate diagnosis.