Differentiation of adrenal adenomas (lipid rich and lipid poor) from nonadenomas by use of washout characteristics on delayed enhanced CT

Background: We describe nonenhanced, early contrast-enhanced, and delayed contrast-enhanced computed tomographic (CT) features and contrast washout characteristics of lipid-poor and lipid-rich adrenal adenomas and nonadenomas to determine the role of these methods in distinguishing one type from the other.Methods: Sixty-five patients with 77 adrenal masses (16 lipid-poor and 37 lipid-rich adenomas and 24 nonadenomas) were consecutively examined with dynamic helical CT. Nonenhanced CT was followed by early enhanced CT at 5, 10, 15, and 30 min delays after administration of contrast material.Results: The difference between the mean nonenhanced and early contrast-enhanced values of the lipid-poor adenomas and nonadenomas was statistically significant, but the ranges of the values were overlapping. The lipid-poor adenomas had lower mean attenuation values than those of nonadenomas on delayed contrast-enhanced scans at each delay time, but these differences did not reach statistical significance. Even though the relative percentage washout of the lipid-poor adenomas was lower than that of lipid-rich adenomas, it was remarkably different from that of the nonadenomas.Conclusions: The absolute or relative percentage washout of contrast material on delayed contrast-enhanced CT is a highly specific test for the differentiation of lipid-poor and lipid-rich adrenal adenomas from adrenal nonadenomas.