Between Blood and Tumor: Retroperitoneal Hematoma Look-Alike Revealed as Diffuse Large B-Cell Lymphoma.

Diffuse large B-cell lymphoma (DLBCL) frequently involves the abdomen and retroperitoneum, where it may present as bulky perirenal soft tissue. Spontaneous retroperitoneal hematoma is an increasingly recognized cause of flank pain and anemia, and can produce similar mass-like collections on computed tomography. Distinguishing these entities is crucial, as misclassification can delay potentially curative systemic therapy or expose patients to non-beneficial invasive procedures aimed at hemorrhage control. We describe a 58-year-old man presenting with progressive left flank and lower quadrant pain, B symptoms, normocytic anemia, acute kidney injury, and mild hypercalcemia. Contrast-enhanced computed tomography demonstrated a large left perirenal retroperitoneal lesion with attenuation in the range expected for acute blood, encasing the kidney and adjacent vasculature and causing mild hydronephrosis. In the context of uncertain minor occupational trauma and mild coagulopathy, a diagnosis of spontaneous retroperitoneal hematoma was favored, and the patient was managed conservatively. Serial hemoglobin measurements showed a progressive decline without hemodynamic instability, prompting catheter-based angiography, which demonstrated no active arterial extravasation or target for embolization. Subsequent renal mass protocol magnetic resonance imaging on hospital day five revealed imaging characteristics more consistent with an infiltrative cellular neoplasm than with evolving hematoma. Image-guided core biopsy established the diagnosis of high-grade B-cell lymphoma with MYC rearrangement, and the patient achieved complete radiologic remission following systemic chemoimmunotherapy. This case illustrates the substantial clinical and radiologic overlap between retroperitoneal DLBCL and spontaneous hematoma, and emphasizes the limitations of relying on single time point computed tomography in isolation. Extensive rind-like perinephric soft tissue, discordance between anemia and hemodynamic status, absence of an angiographic bleeding source, and the presence of systemic symptoms should prompt reconsideration of a presumed hematoma. A low threshold for advanced cross-sectional imaging and timely tissue diagnosis is essential to avoid diagnostic delay, prevent unnecessary invasive procedures, and expedite appropriate oncologic management when lymphoma underlies an apparent retroperitoneal hemorrhage.