Adrenal incidentalomas are defined as adrenal nodules that are discovered incidentally on up to 4% of abdominal CT or MRI scans obtained for other reasons. Although the overwhelming majority of adrenal incidentalomas are benign adrenal adenomas, it is always necessary to determine whether such masses are malignant or pheochromocytomas and whether they secrete excessive cortisol or aldosterone. Patients with an adrenal nodule and any possible manifestation of hypercortisolism should be screened for Cushing syndrome with a plasma ACTH, serum cortisol, and serum DHEAS; patients with a low or low-normal ACTH, a suppressed DHEAS, or a high cortisol should then be assessed with a 1-mg dexamethasone suppression test (see Cushing syndrome). Patients with hypertension are screened for primary aldosteronism with a PRA and serum aldosterone (see Primary Aldosteronism). Adrenal incidentalomas should be assessed for pheochromocytoma if their density is greater than or equal to 10 HU, particularly when their diameter exceeds 3 cm, density exceeds 10 HU, and in patients with hypertension or suspicious symptoms; screening is done with plasma fractionated free metanephrines (see Pheochromocytoma).
The differential diagnosis of an incidentally discovered adrenal mass includes adrenal carcinoma, pheochromocytoma, metastases, lymphoma, myelolipoma, infection, and cysts. When an adrenal incidentaloma larger than 4 cm is detected in a patient without a history of malignancy, it should be resected, unless it is an unmistakably benign myelolipoma, hemorrhage, or adrenal cyst. Masses 3–4 cm may be resected if they have suspicious features (heterogeneity or irregularity). Smaller adrenal incidentalomas are usually observed after endocrine testing. A noncontrast CT scan should be performed to determine the density of the mass. About 99.5% of adrenal pheochromocytomas have a density greater than or equal to 10 HU; patients with adrenal incidentalomas with densities greater than or equal to 10 HU that are not resected require both clinical follow-up and CT follow-up in 6–12 months.