Retroperitoneal hibernoma mimicking a metastatic germ cell tumor: a case report and review of the literature.

BACKGROUND: Hibernomas are very rare fat-derived benign tumors most commonly developed from residual brown fat sited in the interscapular region, the neck, the axillary, and the mediastinal areas. Retroperitoneal location is extremely rare and is usually confused with other malignancies such as liposarcoma and teratoma. We herein described an additional case of retroperitoneal hibernoma and discussed the challenging diagnosis. CASE PRESENTATION: A 32-year-old White North African man presented with a left-sided testicular tumor mass. The testicular ultrasonography showed an ill-defined hypoechoic lobulated left-sided tumor mass with necrosis and calcifications. Abdominal computed tomography scan revealed a left para-aortic lymph node enlargement (17/26 mm). The patient underwent left orchidectomy, and the pathological report concluded that it was a mixed germ cell tumor. Adjuvant chemotherapy was performed. At 4 months, the abdominal computed tomography scan control revealed incomplete regression of the left para-aortic lymph node (10 × 15 mm), as it was thought to be a metastatic lymph node, surgical removal was performed. On pathological examination, the specimen was composed of polygonal eosinophilic brown fat cells admixed with mature unovacuolated adipocytes. No germ cell component, lymphoid tissue, calcifications, or necrosis was found. A final diagnosis of hibernoma was made. CONCLUSION: The present case highlighted the challenging diagnosis of a retroperitoneal mass in the context of a known germ cell tumor. Further case reports are needed to increase awareness among radiologists and surgeons about this entity, which should be included in the differential diagnosis of a retroperitoneal mass in a young patient.