Cushing's syndrome due to a large adrenocortical adenoma with histological features simulating ACTH-independent macronodular adrenocortical hyperplasia

A 53-year-old woman presented with Cushing's syndrome resulting from an adrenocortical adenoma, 6.5 cm in diameter and 75 g in weight, which is larger than usual. Endocrinological data of this patient showed adrenocorticotropin (ACTH)-independent hypercortisolemia. A computed tomography scan of the adrenal glands revealed a single large and well-encapsulated tumor with an irregularly shaped area of calcification and loss of parenchyma on the left adrenal. The right adrenal gland was atrophic. Laparoscopic removal of the left adrenal tumor was performed. The tumor was lobulated and clearly encapsulated, and the non-neoplastic area of the left adrenal was atrophic without any nodularity. The histological analysis confirmed the diagnosis of adrenal adenoma. In addition, this adenoma displayed histopathological features in common with ACTH-independent macronodular adrenocortical hyperplasia (AIMAH), including clear cell predominance, a pattern of small compact cell nests in clear cell areas, and very long cord-like arrangement of small compact cells. In AIMAH, adrenals are extremely enlarged and are more massive than in any other subtype of Cushing's syndrome. The fact that the present adrenocortical adenoma was larger than those typical adenomas of Cushing's syndrome may reflect an AIMAH-type cellular composition of clear cell predominance and small compact cell nests.