When “Metastatic Disease” Is Not Metastatic: Diffuse Large B-cell Lymphoma Presenting With a Perisplenic Mass, Pulmonary Masses, Pleural Effusion, and Retroperitoneal Lymphadenopathy

Diffuse large B-cell lymphoma (DLBCL) is the most common aggressive non-Hodgkin lymphoma and may present with nodal or extranodal disease. Thoracic and abdominal extranodal involvement can produce a radiographic appearance that closely mimics disseminated metastatic solid-organ malignancy, particularly when pulmonary masses, pleural abnormalities, splenic lesions, and bulky abdominal disease coexist. We report the case of an 81-year-old woman who initially presented with wheezing, productive cough, hypoxia, and dysuria and was referred for presumed pneumonia and urinary tract infection. Imaging subsequently revealed multiple bilateral pulmonary and pleural-based masses, bulky mediastinal and hilar lymphadenopathy, a large left upper quadrant/perisplenic mesenteric mass, splenomegaly with multiple splenic lesions, retroperitoneal and mesenteric lymphadenopathy, pleural effusion, and left hydroureteronephrosis due to malignant obstruction. Core needle biopsy of the left upper quadrant mesenteric mass demonstrated high-grade B-cell lymphoma (BCL) most compatible with diffuse large B-cell lymphoma. Immunohistochemistry showed cluster designation (CD)20, CD79a, CD10, and BCL6 positivity with negative BCL2, multiple myeloma oncogene1 (MUM1), and cyclin D1, consistent with a germinal center B-cell-like phenotype by the Hans algorithm, and Ki-67 was approximately 80%-90%. Flow cytometry identified a CD10-positive, kappa-restricted, abnormal mature B-cell population, and fluorescence in situ hybridization later demonstrated BCL2 rearrangement without MYC or BCL6 rearrangements, arguing against double-hit lymphoma. This case highlights an important diagnostic pitfall: apparent widespread metastatic disease may in fact represent aggressive lymphoma, and early tissue diagnosis is essential when the radiographic pattern does not clearly fit a single solid-organ primary.