Clinical meaning
An adrenal incidentaloma is an adrenal mass ≥ 1 cm discovered on imaging performed for indications other than suspected adrenal disease. Prevalence is 3-7% on abdominal CT (increases with age — up to 10% in elderly). Most are non-functioning cortical adenomas (80%). The two critical questions in evaluation are: (1) Is it hormonally active? (subclinical Cushing > pheochromocytoma > primary aldosteronism > adrenal carcinoma; functional tumors require treatment regardless of size), and (2) Is it malignant? (adrenocortical carcinoma is rare but aggressive; size > 4-6 cm, irregular borders, heterogeneous enhancement, washout < 50% on delayed CT are concerning features). CT characteristics guide risk assessment: Hounsfield units (HU) < 10 on unenhanced CT indicates lipid-rich adenoma (benign — no further imaging needed); HU > 10 requires contrast washout study (absolute washout > 60% and relative washout > 40% at 15 minutes suggests adenoma). MRI signal drop on out-of-phase compared to in-phase images (India ink artifact at tumor-tissue interface) indicates intracellular lipid = adenoma.