Intravascular Large B-cell Lymphoma Presenting as Hemophagocytic Lymphohistiocytosis and Pyrexia of Unknown Origin: A Diagnostic Dilemma.

Intravascular large B-cell lymphoma (IVBCL) is a rare, aggressive subtype of diffuse large B-cell lymphoma distinguished by the proliferation of malignant lymphoid cells within the lumina of small vessels. Its unusual growth pattern results in absent lymphadenopathy and nonspecific imaging findings, often leading to delayed diagnosis. We report the case of a 71-year-old man who presented with prolonged fever, hepatosplenomegaly, anemia, leukopenia, thrombocytopenia, and markedly elevated ferritin. He was diagnosed with secondary hemophagocytic lymphohistiocytosis (HLH) and initially improved with HLH-directed immunosuppressive therapy but soon deteriorated again. PET-CT showed only diffuse hepatic and splenic uptake without discrete lesions. A bone marrow biopsy revealed IVBCL. This case highlights the diagnostic challenges of IVBCL, particularly its overlap with HLH, the limitations of PET-CT, and the crucial role of bone marrow biopsy. Early detection and initiation of therapy remain essential for improving outcomes.