Renal Mucosa-Associated Lymphoid Tissue Lymphoma Mimicking Autoimmune Pancreatitis: A Diagnostic Challenge.

An 87-year-old man presented with progressive steatorrhea, weight loss, and subsequent hyperglycemic crisis. Initial workup revealed combined exocrine and endocrine pancreatic insufficiency with mildly elevated immunoglobulin G4 below the diagnostic threshold and diffuse pancreatic inflammation on magnetic resonance imaging, raising suspicion for autoimmune pancreatitis. Follow-up imaging revealed an incidental renal mass, which biopsy confirmed as mucosa-associated lymphoid tissue lymphoma. Positron emission tomography-computed tomography demonstrated fluorodeoxyglucose uptake in both the renal mass and pancreatic tissue, with concordant treatment response to rituximab in both locations, suggesting lymphomatous pancreatic infiltration.