Retroperitoneal Hibernoma Presenting as a Gastrointestinal Surgical Diagnostic Challenge: A Rare Benign Lipomatous Tumor and Literature Review.

Background: Hibernoma is an uncommon benign adipocytic neoplasm showing differentiation toward brown fat. Although it usually occurs in anatomical sites where residual brown adipose tissue persists, retroperitoneal involvement is particularly uncommon, with fewer than 60 cases reported. Case Presentation: A 54-year-old man was found to have a retroperitoneal mass incidentally during a routine physical examination. CT and MRI demonstrated a 15-cm homogeneous fat-containing lesion without radiological evidence of local invasion. Complete surgical excision was performed. Histopathological examination showed univacuolated and multivacuolated adipocytic cells without atypia. The tumor was positive for S-100 and CD10, and fluorescence in situ hybridization showed no MDM2 amplification, supporting the diagnosis of hibernoma. Recovery was uneventful, and the patient was discharged on Postoperative Day 7. Discussion: Retroperitoneal hibernoma can be difficult to distinguish radiologically from other lipomatous neoplasms, including atypical lipomatous tumor/well-differentiated liposarcoma (ALT/WDL). Diagnosis therefore relies on the integration of morphology with ancillary immunohistochemical and, when indicated, molecular findings. The decision between preoperative core needle biopsy and upfront resection should be individualized according to the radiological differential diagnosis, tumor vascularity and resectability, expected surgical morbidity, and whether tissue confirmation would alter management. Conclusion: Hibernoma should be included in the differential diagnosis of retroperitoneal fat-containing masses. Accurate diagnosis is fundamental, as complete surgical excision is curative, with no need for additional treatment.