Can Established CT Attenuation and Washout Criteria for Adrenal Adenoma Accurately Exclude Pheochromocytoma?

Abstract

The purpose of this article is to determine the proportion of pheochromocytomas that mimic adrenal adenoma using established CT washout and attenuation criteria. The CT characteristics of pheochromocytomas confirmed by histologic analysis (n = 46) and (131)I-metaiodobenzylguanidine (n = 1) were compared with those of 98 adrenal adenomas (negative plasma and urinary metanephrines or catecholamines, and one or more of the following characteristics: unenhanced attenuation ≤ 10 HU, absolute washout ≥ 60%, and relative washout ≥ 40%). CT numbers were measured in all available phases (unenhanced [n = 37], 1-minute contrast enhanced [n = 46], and delayed contrast enhanced [n = 43]) using a region of interest that encompassed the majority of the mass. Absolute washout, relative washout, and degree of enhancement (1-minute minus unenhanced) were calculated. Mass size and heterogeneity were recorded and compared using the Student t test and a chi-square test, respectively. Twenty-four of 47 (51%) pheochromocytomas were imaged with a triphasic examination using a 15-minute delay. Eight of 24 (33%) met relative (6/24 [25%]) or absolute (7/24 [29%]) washout criteria for the diagnosis of a lipid-poor adenoma. Four of these (50% [4/8]) were homogeneous on all three phases. None of the pheochromocytomas had an unenhanced attenuation of 10 HU or less. Pheochromocytomas were significantly larger than adrenal adenomas (mean diameter, 3.9 cm [range, 0.6-14 cm] vs 2.0 cm [range, 0.8-3.9 cm]; p < 0.0001) and were significantly less likely to be homogeneous (15/47 [32%] vs 95/98 [97%]; p < 0.0001), but there was overlap. A substantial minority of pheochromocytomas have absolute or relative washout characteristics that overlap with those of lipid-poor adenomas.