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Background:The differential diagnosis for retroperitoneal masses is broad, and includes pancreatic lipomatous hamartoma, a rare but benign disease process. Hamartoma describes outgrowth of tissue native to the organ in which it occurs and is considered a malformation rather than a true neoplasm. Pancreatic hamartomas consist of less than 1% of all hamartomas, and few cases of pancreatic lipomatous tumors with distinct components of pancreatic hamartoma have been described. These tumors are difficult to diagnose on imaging and biopsy alone and can appear malignant with potential for significant mass effect on surrounding organs, leading to surgical intervention for definitive diagnosis. Case Presentation:A 57-year-old male presented with right upper quadrant pain radiating to his left side and back. Ultrasound demonstrated cholelithiasis and an incidentally discovered large retroperitoneal mass. Computed tomography (CT) imaging demonstrated a 14.4 x 4.6 x 16 cm heterogenous, fatty-appearing retroperitoneal mass displacing the colon and small bowel, while abutting the aorta and inferior vena cava (IVC) with no obvious invasion of surrounding structures. He underwent laparoscopic cholecystectomy with biopsy of the mass. Biopsy demonstrated benign adipose tissue and no evidence of malignancy, and his care was transferred to our facility. His postoperative course was complicated by acute congestive heart failure that ultimately delayed definitive surgical intervention for nine months. Progressive growth of the mass was noted on serial imaging as his cardiopulmonary status was optimized. Preoperative CT revealed a 20.2 x 28.2 x 14.1 cm with worsened mass effect on surrounding structures.The patient underwent open resection of the retroperitoneal mass with en bloc resection of the third and fourth portions of the duodenum and the uncinate process of the pancreas. The final pathologic diagnosis was benign lipomatous proliferation with features most consistent with pancreatic lipomatous hamartoma (PLH), with specimen weight of 3810g and overall dimensions of 35 x 27 x 14 cm.Conclusions:PLH is an extremely rare benign tumor, which is notoriously difficult to diagnose with imaging and biopsy alone. This case highlights the difficulty delineating benign from malignant tumors of the pancreas and retroperitoneum.