Most adrenal masses are incidentally found during a CT scan. They are common, occurring in up to 2% of the population.

The differential diagnosis of an adrenal mass includes both benign and malignant processes. The majority will be benign, especially in patients with no history of malignancy. An incidental and presumably benign adrenal mass has been refered to as an "incidentaloma". However, in a patient with a preexisting malignancy, the concern for metastatic disease is much greater. Up to 40% of adrenal masses found on CT in patients with known malignancy can represent metastatic disease. The patient in the current case fits into this category (history of renal cell carcinoma).

Virtually any primary can metastasize to the adrenal but lung, breast, and renal primaries are perhaps most common. Melanoma and lymphoma can also involve the adrenals.

Primary adrenal tumors include pheochromocytoma, neuroblastoma, ganglioneuroma, hyperfunctioning adenomas (Conn syndrome, Cushing syndrome, and virilizing tumors), nonhyperfunctioning adenomas, myelolipoma, cysts (epithelial and pseudocysts) and cortical carcinoma. Nonneoplastic hyperplasia (congenital or acquired) typically causes bilateral adrenal enlargement but can be asymmetric or mass-like.

Most imaging features of adrenal tumors are nonspecific. Lesions that are 5 cm and greater are usually removed surgically.

Fortunately, there is a way to distinguish benign adenomas from malignant tumors without biopsy. This is because adenomas usually contain a high concentration of cytoplasmic lipid, which is almost never seen with adrenal malignancies. It is possible to distinguish lipid rich tumors from lipid poor tumors using CT attenuation coefficients (Hounsfield units - HU). Fat containing structures will lower the Hounsfield units more than other soft tissues.

Threshold HU levels on noncontrast CT have been developed that permit separation of benign and malignant tumors of the adrenal gland. A measurement below the threshold indicates benign disease whereas a measurment above the threshold is considered indeterminate and further investigation needs to be undertaken to classify the tumor.

The various threshold levels have corresponding sensitivities and specificities. A threshold of 2 HU is virtually 100% specific for an adenoma. In other words, all adrenal masses with HU less than 2 are benign. However, this has relatively low sensitivity and many adenomas will be considered indeterminate and undergo biopsy. However, if one is willing to accept a small false negative rate, a threshold of 10 HU can be used. This yields a still very high (96%) specificity rate and an acceptable sensitivity of 73%.

Following IV contrast, adenomas enhance and their HU levels greatly overlap with other lesions. More useful, however, is the washout curve following IV contrast. Adenomas demonstrate faster washout than metastatic lesions and this information may prove useful when noncontrast images are not available. Further research is currently underway.

Finally, an MR imaging technique called chemical shift imaging also takes advantage of the cytoplasmic lipid in adenomas. Gradient echo images are obtained when fat and water protons are in phase and also when they oppose each other and cancel out. Signal drop-out is seen in adenomas on the opposed-phase images but not with metastatic lesions. The accuracy of this technique rivals that of CT.