122 AJR:201, July 2013 nal cortical carcinoma incidence is 0.72 cases/ million persons/year [12]). Although Szolar et al. [8] showed that the washout profile of ad- renal adenomas using a 10-minute delay may be significantly different from that of primary adrenal tumors, their work was criticized for its small sample size [13]. To this point, there is some evidence that adrenal pheochromocy- tomas—rare neuroendocrine tumors that typi- cally arise from chromaffin cells—can exhibit imaging characteristics similar to those of ad- renal adenomas, including unenhanced attenu- ation values less than or equal to 10 HU [14], absolute contrast material washout 60% or higher [7, 14–17], or relative contrast material washout 40% or higher [14, 17]. Because pheochromocytomas can be both clinically significant and clinically silent [18, 19], the ability to differentiate pheochromocy- toma from adrenal adenoma is a worthwhile goal. Inaccurate characterization could have Can Established CT Attenuation and Washout Criteria for Adrenal Adenoma Accurately Exclude Pheochromocytoma? Jeet Patel 1 Matthew S. Davenport Richard H. Cohan Elaine M. Caoili Patel J, Davenport MS, Cohan RH, Caoili EM 1 All authors: Department of Radiology, University of Michigan Health System, 1500 E Medical Center Dr, B2 A209P, Ann Arbor, MI 48109. Address correspondence to M. S. Davenport (matdaven@med.umich.edu). Genitourinary Imaging • Original Research AJR 2013; 201:122–127 0361–803X/13/2011–122 © American Roentgen Ray Society D edicated adrenal-protocol CT us- ing a triphasic examination com- posed of unenhanced, enhanced (1-minute contrast enhanced), and delayed enhanced (15-minute contrast en- hanced) imaging has been established as a spe- cific method for differentiating adrenal adeno- mas from nonadenomas [1, 2]. This practice has been validated in multiple studies [3–9]. Because of its ability to noninvasively provide a histologic surrogate for incidentally discov- ered adrenal nodules, adrenal-protocol CT has gained widespread acceptance [10, 11]. Investigations establishing the contrast en- hancement time–washout profile for nonad- enomas have relied largely on data from ad- renal metastases in patients with extraadrenal malignancies [3–7]. Primary adrenal tumors have received less attention because of their rarity (i.e., the pheochromocytoma incidence is 8 cases/million persons/year, and the adre- Keywords: absolute washout, adrenal adenoma, CT washout, pheochromocytoma, relative washout DOI:10.2214/AJR.12.9620 Received July 18, 2012; accepted after revision September 6, 2012. R. H. Cohan is a consultant for a legal firm representing GE Healthcare in ongoing nephrogenic systemic fibrosis litigation. OBJECTIVE. The purpose of this article is to determine the proportion of pheochromo- cytomas that mimic adrenal adenoma using established CT washout and attenuation criteria. MATERIALS AND METhODS. The CT characteristics of pheochromocytomas con- firmed by histologic analysis (n = 46) and 131 I-metaiodobenzylguanidine (n = 1) were com- pared with those of 98 adrenal adenomas (negative plasma and urinary metanephrines or cat- echolamines, 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 unen- hanced) 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 unen- hanced 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 rela- tive washout characteristics that overlap with those of lipid-poor adenomas. Patel et al. Exclusion of Pheochromocytoma Using CT Attenuation and Washout Criteria Genitourinary Imaging Original Research Downloaded from www.ajronline.org by 52.73.204.196 on 05/16/22 from IP address 52.73.204.196. Copyright ARRS. For personal use only; all rights reserved