Teaching File Case - Patient: 2756New Submission - Pending Editor Assignment
 |
To complete the differential, there are other cancers that metastasize to the adrenal gland. They include lung and breast cancer (most common), melanoma and GI cancer (less common). In fact, adrenal metastasis is very common, occurring in up to 27% of patients with malignant disease.
Primary adrenal tumors include pheochromocytoma, neuroblastoma, ganglioneuroma, aldosteroneoma, myelolipoma, other adenomas (functional and non-functional), cysts, and carcinoma. Non-neoplastic hyperplasia (congential or from elevated ACTH) could also look like an adrenal mass but in the setting of a unilateral abnormality would be unlikely.
Adrenal Adenoma While morphological characteristics can characterize an adrenal tumor more clearly, these features are nonspecific. For example, a larger or irregular lesion is more likely malignant, however, small, smooth lesions may also be malignant. Furthermore, tumor morphology cannot help to distinguish metastatic from non-metastatic disease.
Fortunately, there is a way to distinguish most benign from malignant tumors without biopsy. This is because the adrenal gland, and many benign adrenal tumors contain a high amount of intracytoplasmic lipid. Furthermore, almost no adrenal malignancies contain high amounts of intracytoplasmic fat.
It is possible to distinguish lipid rich tumors from lipid poor tumors using CT attenuation coefficients (Hounsfield units). Fat containing structures will lower the Hounsfield units more than other soft tissues.
With non-contrast CT, thresholds have been developed that permit separation of benign from malignant tumors of the adrenal. Masses with HU below the threshold are considered benign and are followed for increase in size.
Masses with HU above the threshold are considered indeterminate and further investigation needs to be undertaken to classify the tumor.
These thresholds have corresponding sensitivities, and specificities. The threshold of 2 HU has virtually 100% specificity 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%.
Attempts have been made to establish thresholds for contrast-enhanced CT. This would be valuable since most adrenal masses are detected in CTs with IV contrast. This has proved problematic, however, because adrenals enhance with IV contrast. This results in benign masses enhancing to attenuation values close to the attenuation values of malignant tumors. Therefore, non-contrast CT should be used to characterize an adrenal lesion.
MRI chemical shift techniques can also differentiate benign from malignant masses in this way. However, CT is more widely available, quicker, and less expensive. Nevertheless, MR1 has usefulness in situations where ionizing radiation needs to be avoided (pregnancy).
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.
