A Critical Analysis of Computed Tomography Washout in Lipid-Poor Adrenal Incidentalomas.

Background: Contrast-enhanced computed tomography (CT) with washout has emerged as an option to distinguish lipid-poor adenomas from non-adenomas.

Objective: The aim of this study was to assess the utility of CT washout in characterizing indeterminate lipid-poor adrenal incidentalomas.

Methods: From an Institutional Review Board-approved database, patients with adrenal incidentalomas who had adrenal protocol CT scans with a 15-min washout between 2003 and 2019 were identified. Non-contrast CT attenuation and washout patterns of different tumor types were compared.

Results: Overall, 156 patients with 175 adrenal lesions were included. Average tumor size was 3.0 cm, non-contrast CT density was 24.7 Hounsfield units (HU), and absolute washout was 52.6%. In 102 lesions (58.3%), CT washout was ≥ 60%; 94 (92.2%) of these were benign adrenocortical adenomas, 7 (6.9%) were pheochromocytomas, and 1 (0.9%) was an adrenal hematoma. Furthermore, in 73 tumors (41.7%), CT washout was < 60%; diagnosis was benign adrenocortical adenoma in 45 (61.6%) lesions, pheochromocytoma in 8 (11%) lesions, metastasis in 9 (12.3%) lesions, adrenocortical cancer in 6 (8.2%) lesions, and 'others' in 5 (6.9%) lesions. Sensitivity, specificity, positive predictive value, negative predictive value, and accuracy of  > 60% absolute CT washout for detecting an adrenal adenoma was 67.6%, 77.8%, 92.2%, 38.4%, and 69.7%, respectively.

Conclusion: CT washout should be incorporated into the management algorithm of indeterminate adrenal incidentalomas with a high non-contrast CT attenuation to 'rule-in' benign tumors. For small tumors with mild elevation of plasma metanephrines, it should be kept in mind that adenomas and pheochromocytomas may have similar imaging and washout characteristics.