Objective: The purpose of this article is to determine the proportion of pheochromocytomas that mimic adrenal adenoma using established CT washout and attenuation criteria.
Materials and methods: 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.
Results: 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.
Conclusion: A substantial minority of pheochromocytomas have absolute or relative washout characteristics that overlap with those of lipid-poor adenomas.