Fever of unknown origin is always a diagnostic challenge in establishing etiology. A gentleman in his 70s presented with complaints of fever and dry cough for 2 months duration. We proceeded with contrast imaging of the thorax and abdomen which revealed mild hepatomegaly. Bone marrow examination with bone marrow culture and 18-fluorodeoxyglucose positron emission tomography/computed tomography (18-FDG PET/CT). Bone marrow aspirate smears were hypercellular with around 50% atypical lymphoid cells with plasmacytoid and bizarre morphology with few multilobed nuclei. Bone marrow biopsy revealed predominantly sinusoidal involvement by neoplastic cells. On immunohistochemistry, tumor cells were positive for CD45, CD79a, CD20, and MUM1 and were negative for CD5, CD10, and BCL6. Ki-67 was around 60% in tumor cells. FDG PET/ CT revealed diffusely increased uptake in the both axial and appendicular skeleton with (SUVmax 5.06) and diffusely increased FDG uptake (SUVmax 3.67) noted in the spleen. As intravascular large B-cell lymphoma is a highly aggressive non-Hodgkin lymphoma with a high risk of central nervous system involvement, we treated it with chemoimmunotherapy (R-CHOP) with intrathecal methotrexate. After a clinical follow of 3 months, the patient developed relapsed with a soft-tissue swelling over the right leg. The patient was treated with two cycles of R-DHAP and had progressive disease and started on Ibrutinib, Lenalidomide, and Rituximab (2 cycles). Post two cycles, the patient had progressive disease and switched to acalabrutinib based therapy. After 1 month of acalabrutinib-lenalidomide-rituximab therapy, the patient had disease progression and succumbed to the disease.

No abstract