A 49-year-old male visited the hospital because he had gained 2 kg over 4 months and had symptoms of truncal obesity and also facial edema that had progressed for 2 months. There were no abnormalities on the results of a blood test, a serum electrolyte test, and a biochemical test. Twenty-four hour urinary free cortisol (UFC) was increased by 722.18 g (58-403 g) per day and did not decrease with the low-dose dexamethasone suppression test. Cushing syndrome was therefore diagnosed. An abdominal computed tomography (CT) scan was performed to locate the lesion and a 7.7 cm tumor accompanied by internal necrosis was observed on the left adrenal gland (Fig. 1). Positron emission tomography (PET) and CT were performed to ascertain whether the tumor was malignant and was metastasized to other organs. As a result, a hypertrophy was observed of the left adrenal lump but lesions suspicious of metastases were not found (Fig. 2). In the results of several tests performed to assess adrenal function, blood epinephrine, norepinephrine, vanillylmandelic acid, and total metanephrine were within normal levels.

We diagnosed the case as Cushing syndrome caused by the adrenal tumor and performed laparotomy and adrenalectomy because the malignant tumor could not be excluded clinically. The tumor did not seem to be metastasized to other organs at the time of the operation, and 7 days after undergoing the operation, the patient was discharged without intraoperative or postoperative complications.

The extracted lump weighed 260 g and measured 10.0×7.5×4.7 cm. At the incisal surface, the tumor was yellowish and was surrounded by a well-defined capsule; on the inside, necrosis and hemorrhage were partially observed (Fig. 3). Light microscopy showed that the tumor was surrounded by a fibrous pseudocapsule, and adrenocortical necrosis was seen in patches. Also, the tumor tissue consisted of polygonal cells, having abundant eosinophilic cytoplasm, in a regular form. Most nuclei were similar in size, but some were atypically large (Fig. 4). Electron microscopy showed that the oncocytic cytoplasm was filled with a great number of mitochondria; also, some lysosomes, Golgi bodies, small lipid particles, and glycogen particles were observed as is usual with oncocytomas. Likewise, inclusion bodies of high electron density were observed in mitochondria (Fig. 5).

An oncocytoma is a tumor consisting of polygonal cells that abundantly have eosinophilic granule cells, i.e., oncocytes. The oncocyte is defined as a cell in which abnormal mitochondria, which exclude other structures, accumulate. Mitochondrial accumulation and tumorigenesis have not been clarified yet, but there is a possibility that it is related to the inflammatory response, regression, or cellular aging. Many scientists infer that mitochondrial accumulation compensatorily causes functional loss. Oncocytomas develop in various organs, but are especially frequent in epithelial cells of high metabolic activity. Namely, they mostly develop in the salivary gland, the kidney, the thyroid gland, the parathyroid gland, and the hypophysis.1

Adrenocortical oncocytomas have rarely been reported; just 25 cases and 4 cases, inclusive of the present case, have been reported in the foreign literature and the domestic literature, respectively.2-4 With regard to functioning oncocytomas, no cases have been reported in the nation. Even in other countries, just 5 cases have been reported:5-9 namely, one case where androgenic hormone was secreted in a female, one case where interleukin-6 was produced, and 3 cases of Cushing syndrome. The present case indicates Cushing syndrome accompanied by truncal obesity and facial edema.

In most cases, the adrenal tumor is incidentally found in the process of a health examination, and after that, the patient visits a urologist. In this case, however, the patient visited the hospital because of symptoms of Cushing syndrome, and the adrenal tumor was found in process of the examination. The possibility of a metastatic tumor was ruled out because primary lesions were not observed on CT or PET, and the case was diagnosed as Cushing syndrome by assessing adrenal function.

The light microscopy and electron microscopy performed after the operation showed typical characteristics of oncocytomas as reported elsewhere.9,10 The cellular structure of the oncocytoma was compared with other adrenal tumors by using an electron microscope. In the case of the pheochromocytoma, a number of granules were bound to the cytomembrane of high electron density in neurosecretory granules and tumor cells. Benign adrenal adenomas are not filled with mitochondria. Such characteristics distinguish the oncocytoma from others.

Nonfunctioning adrenal tumors, which are incidentally found, are open to dispute. On the other hand, surgical operations should be performed on functioning adrenal tumors. In addition, medical scientists should further study functioning oncocytomas and be more aware of them, in consideration of their rareness.