Adenomatoid tumor of the adrenal gland with ultrastructural and immunohistochemical demonstration of a mesothelial origin.

We describe an adenomatoid tumor arising in the left adrenal gland of a patient who underwent bilateral adrenalectomy for the ectopic adrenocorticotropic hormone syndrome. The mesothelial origin of the tumor was confirmed by electron microscopy and positive immunohistochemical staining for keratin. The tumor had a prominent cystic component raising the question whether adrenal cysts are derived from mesothelial inclusions. It is proposed that consideration be given to adding a new category of "mesothelial cysts" to the classification of adrenal gland cysts.