Differentiation of adrenal adenomas from nonadenomas using CT attenuation values

The purpose of our study was to determine whether unenhanced CT attenuation value, enhanced CT attenuation value, or lesion size can be used to differentiate adrenal adenomas from nonadenomatous adrenal masses.

We retrospectively assessed the CT scans of 135 adrenal masses in 124 patients with a variety of adrenal masses. There were 93 cortical adenomas (85 nonhyperfunctioning adenomas, four Cushing's adenomas, and four primary aldosteronism adenomas). The nonadenomas consisted of 34 metastases, four cortical carcinomas, and four pheochromocytomas. The scattergrams and mean values of the size and attenuation values on enhanced and unenhanced scans were correlated with the final diagnoses. Results were also subjected to receiver operating characteristic analysis.

Forty-one adenomas and 20 nonadenomas had unenhanced CT. The mean attenuation value of the 41 adenomas was significantly lower (p < .001) than that of the nonadenomas (2.5 H +/- 14 compared with 32 H +/- 6.4). The lowest unenhanced CT attenuation value of the nonadenomas was 18 H; therefore, the sensitivity:specificity ratio for the diagnosis of adenomas was 85%:100% at a threshold value of 18 H. At this threshold, the positive predictive value was 100% and the negative predictive value was 77%. For the 85 masses with enhanced CT, the mean attenuation of the 60 adenomas was also significantly lower (p < .01) than for the 25 nonadenomas (47 H +/- 24 compared with 62 H +/- 21). The lowest enhanced CT attenuation value of the nonadenomas was also 18 H, but the sensitivity:specificity ratio was only 10%:100% at this threshold value of 18 H. Although the mean diameter of the adenomas was significantly lower (p < .001) than for the nonadenomas (2.4 cm +/- 0.9 compared with 4.5 cm +/- 2.5), there was sufficient overlap between the two groups at the smallest sizes that a threshold value for a highly specific diagnosis of adenoma was not present. The area under the receiver operating characteristic curve for unenhanced CT attenuation values (0.98 +/- 0.02) was significantly greater than the area for enhanced CT values (0.68 +/- 0.06, p < .001) and the area for size (0.79 +/- 0.04, p < .001).

Unenhanced CT attenuation values can characterize an adrenal mass as a benign adenoma with high specificity and acceptable sensitivity. Adrenal masses cannot be characterized using enhanced CT attenuation values or lesion size.