Answer: Metastatic renal cell carcinoma

Histology: The lesion consists of a circumscribed area of clear cells growing in a nested and tubular pattern. The nuclei show some variation in size and shape, some with nucleoli. Compared to the adjacent normal adrenal tissue, the tumor cells are clear rather than vacuolated and granular.

Discussion: The lesion in the adrenal raises a differential diagnosis of primary adrenal neoplasm and metastatic lesions to the adrenal gland. The lack of necrosis, nuclear anaplasia and vascular invasion, and the small size of the lesion make an adrenal cortical carcinoma unlikely. The clear cells raise the possibility of a metastasis from the kidney, which would be important to differentiate from an adrenal cortical adenoma. To unequivocally establish the diagnosis, immunohistochemical stains need to be performed. RCC, CD10, CAM 5.2, EMA are all expected to be positive in renal cell carcinoma and negative in adrenal cortical lesions, while inhibin and Melan-A should be positive in adrenal cortical lesions and negative in renal cell carcinoma. Pheochromocytomas arise from the adrenal medulla and usually shows pleomorphic cells with basophilic cytoplasm, unlike the clear cells seen in this case, neuroendocrine markers, such as chromogranin would also be positive, in contrast to the other entities in the differential.

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