Adrenocortical carcinoma (ACC) is a rare aggressive tumor originating from adrenocortical parenchymal cells and its incidence is approximately 1 per million population per year. An oncocytic ACC is a recently identified entity among the several known histopathological variants of ACC, which is characterized by oncocytic cells, and only a few cases in the available literature have reported this tumor. In contrast to conventional ACCs, oncocytic ACCs usually manifest as solitary lesions presenting in adults without any sex predilection. We report a case of a 70-year-old Japanese man who presented with an incidentally discovered retroperitoneal mass without any evidence of excessive corticosteroid secretion. Laboratory and imaging studies, as well as transgastric endoscopic ultrasound-guided fine needle aspiration failed to establish a definitive diagnosis. Thus, the patient underwent surgical resection of the left-sided peritoneal tumor. Weiss score was positive in 6/9 points and the tumor met two major criteria of the Lin-Weiss-Bisceglia (LWB) system leading to a diagnosis of an oncocytic variant of ACC. Based on our findings in this patient, we conclude that a combination of the Weiss and LWB criteria is required to determine the malignant potential of oncocytic adrenal tumors because ACCs and oncocytomas could be frequently indistinguishable. Careful histopathological examination is pivotal in confirming the oncocytic component in the lesion and hence definitive diagnosis of ACCs.
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