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). Most adrenal incidentalomas should be assessed for pheochromocytoma, 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 includes adrenal carcinoma, pheochromocytoma, metastases, lymphoma, myelolipoma, infection, and cysts. When an adrenal incidentaloma larger than 4 cm in diameter 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 in diameter may be resected if they have suspicious features (heterogeneity or irregularity). Smaller adrenal incidentalomas are usually observed after endocrine testing. A noncontrast CT scan can determine the density of the mass; adrenal incidentalomas with a density less than 10 HU on CT are unlikely to be a pheochromocytoma or metastasis. For small adrenal incidentalomas with a density of greater than or equal to 10 HU, an adrenal intravenous contrast “washout” CT scan may be obtained; the density of the adrenal incidentaloma in HU is calculated 60 seconds after contrast and again 15 minutes after contrast; a reduction (washout) of 40% or more is consistent with a benign adrenal adenoma. However, adrenal washout studies are only suggestive and about 20% of pheochromocytomas have adrenal washout of 40% or more; so, if the adrenal incidentaloma is not clinically a pheochromocytoma and not resected, a follow-up CT of the adrenals in 6–12 months is recommended to look for growth.
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