Is biochemical screening for pheochromocytoma in adrenal incidentalomas expressing low unenhanced attenuation on computed tomography necessary?

OBJECTIVE: Pheochromocytomas are characterized by a high attenuation value on unenhanced computed tomography (CT). It is not known whether pheochromocytoma could be ruled out as a cause of adrenal incidentalomas on the basis of unenhanced attenuation values only.
DESIGN: We retrospectively evaluated the outcome of routine biochemical screening for pheochromocytoma in a series of adrenal incidentalomas in relationship to the unenhanced attenuation values on CT.
METHODS: An unenhanced CT was available in 174 of 184 patients with 214 adrenal incidentalomas. All patients were screened for pheochromocytoma with 24-h urinary metanephrines and normetanephrines and for hypercortisolism (1 mg dexamethasone test and ACTH). Hypertensive patients were screened for aldosterone overproduction (aldosterone to renin ratio and 24 h urinary aldosterone). The results were compared between incidentalomas with high [≥10 Hounsfield units (HU)] and low (<10 HU) unenhanced attenuation values.
RESULTS: One hundred forty-six incidentalomas in 115 patients had an unenhanced HU less than 10. None of these patients had elevated 24-h fractionated urinary metanephrines or normetanephrines suggesting pheochromocytoma. Sixty-eight incidentalomas in 59 patients had an unenhanced HU of 10 or greater, and nine (15.2%) of these patients had surgically and histologically verified pheochromocytoma. Incidentalomas with a HU of 10 or greater were significantly larger (2.6 ± 1.5 vs. 2.3 ± 1.2 cm; P < 0.001), more often functional (27.9 vs. 8.9%, P < 0.001), and more often operated (44.1 vs. 10.2%; P < 0.001) than those with a Hounsfield unit less than 10.
CONCLUSION: The results of this study indicate that routine biochemical screening of pheochromocytoma in small homogenous adrenal incidentalomas characterized by an unenhanced Hounsfield unit value less than 10 HU may not be necessary.