CT Criteria for Adenoma

Several radiologic criteria can be used to diagnose an adenoma. The most well-known technique is to measure the mean attenuation value of the lesion. The rationale for this technique is based on a histologic feature of adenomas—that of abundant intracytoplasmatic lipid in the cortex. In contrast, malignant tumors of the adrenal gland have relatively little intracytoplasmic lipid, which can be detected by CT and MRI. A high correlation between lipid content and density measurements on CT has been shown, and when attenuation levels are between 10 HU and 15 HU or less on thin-section non-enhanced CT images, the diagnostic specificity approaches 100% . Although nonenhanced CT has a very high specificity, sensitivity is low, and higher attenuation values do not exclude the diagnosis of adenoma

Unfortunately, most CT examinations of the abdomen are performed after IV contrast, particularly if the indication is to look for metastatic disease, and mean attenuation measurements of adrenal adenomas may often be greater than 10 to 15 HU on postcontrast images. In this instance, looking at the contrast washout pattern of the adrenal lesion may help to diagnose an adenoma.

Studies have shown that adenomas show rapid washout of contrast when compared with nonadenomas and that this characteristic can be detected on delayed images acquired 10 to 15 minutes after the administration of IV contrast (Figure 10). If a relative washout of 40% to 50% is demonstrated, the test has a sensitivity of 83% to 93% and specificity of 93% to 98% for the diagnosis of an adenoma.An absolute washout of 60% is also highly specific and sensitive to make this differentiation. These parameters can be used to differentiate lesions that are nonspecific on nonenhanced CT (lipid-poor adenomas), as well as to characterize lesions identified only after the administration of IV contrast, when the use of a threshold of 10 HU or 15 HU is less likely to be useful for the diagnosis of adenoma.

Recently, a new method to detect the presence of fat within adrenal lesions was described by Bae et al. This study found that an accurate diagnosis of adenoma could be made using a histogram analysis method consisting of selecting a region of interest (ROI) within the adrenal mass and looking at the histogram distribution of pixel attenuation values. In this study, the presence of at least 10% negative pixels (ie, 10% lipid pixels) provided a specificity of virtually 100% for diagnosing adenoma. Although the sensitivity of this method was not high (reported to be 28%), the technique was found to have potential application in cases in which IV contrast was administered but delayed images were not available.