Nonhyperfunctional adrenal adenomas are common, representing an incidental finding in as much as 2-8% of autopsies. The most common cause of an incidentally discovered adrenal mass on imaging is a benign, nonhyperfuntioning adenoma. These must be differentiated from an unsuspected metastasis or a rare, small, adrenal carcinoma. Incidental discovery of an adrenal metastasis as the first manifestation of an occult primary tumor is a rare event.

On CT, benign adenomas are typically small (<4cm), well-defined, smooth, round, and homogeneous. Because benign adenomas are rich in lipids, they tend to be low in CT density. On a nonenhanced CT scan, a measured CT density of less than 10 is strongly predictive of benign adenoma and virtually excludes a metastatic lesion (73% sensitive + 96% specific for adenoma). Benign adrenal adenomas enhance uniformly with intravenous contrast administration.

The pharmacokinetics of IV contrast enhancement of lipid-poor adrenal adenomas may be useful for differentiating these from malignancy using CT. IV contrast enhancement of lipid-poor adrenal adenomas appears to be identical to that of lipid-rich adenomas and significantly different from that of adrenal nonadenomas. Caoili et all reported a highly significant difference between the two groups (p < 0.001) for the mean contrast enhancement, enhancement washout, and percentage enhancement washout. The mean percentage enhancement washout of the lipid-poor adenomas was also statistically identical to that of the lipid-rich adenomas and was significantly greater than that of the nonadenomas. They demonstrated a specificity of 95% for both lipid-poor and lipid-rich adenomas using an optimal threshold percentage enhancement washout of 60%., as well as sensitivities of 89% and 79% for the lipid-poor and lipid-rich adenomas. Using the optimal threshold of 40%, the relative percentage enhancement washout for lipid-poor adenomas resulted in a sensitivity of 83% and a specificity of 93%. For the lipid-rich adenomas, a relative percentage enhancement washout threshold of 50% was associated with a sensitivity of 93% and specificity of 98%,

Washout percentages were calculated using the following equations: percentage enhancement washout = (enhancement washout / enhancement) x 100. The relative percentage enhancement washout = (enhancement washout / enhanced attenuation value) x 100.

Chemical shift MR imaging is used to identify benign adrenal adenomas, and thereby exclude the presence of a metastatic lesion by demonstrating the higher lipid content of benign adenomas. MRI is even more sensitive than CT to the presence of lipid. Two separate T1 weighted or gradient echo pulse sequences are obtained, one with the water and fat resonance peaks aligned, and the other with them completely out of alignment. A reduction of the signal intensity on the out-of-phase image, using the spleen as the internal reference, is evidence of lipid in the tumor and confirms benign adenoma. Increased or unchanged signal on the out-of-phase image compared with the in-phase image indicates a lipid-poor tumor. This is a nonspecific finding, as is the finding of a mass that demonstrates greater than 10 HU on a noncontrast CT , since a significant percentage of adenomas are lipid-poor.