Diffuse Large B-cell Lymphoma in a Patient With Refractory Perinephric Fluid Collection and Pleural Effusion: A Case Report.

Perinephric fluid collections have a broad differential diagnosis and may be difficult to characterize by imaging or fluid analysis. Diffuse large B-cell lymphoma (DLBCL) typically presents as mass-like lesions in nodal or extranodal sites but may rarely present as isolated fluid collections. We report a 53-year-old woman with acute right flank pain after minor trauma. Imaging showed a large right perinephric fluid collection extending into the retroperitoneum and pleural cavity without an identifiable mass. Fluid studies and lymphangiography did not identify a clear underlying etiology. Despite intervention for persistent high-output drainage, the patient developed prolonged fever and was ultimately diagnosed with DLBCL following biopsy of a newly developed cheek lesion. This case highlights an atypical presentation of DLBCL in a patient with a persistent perinephric fluid collection and underscores the importance of considering malignancy in unexplained, refractory fluid collections.