Adrenal adenoma

Discussion of Adrenal adenoma

Adrenal adenoma is a common tumor, with a prevalence of 3% in autopsy series.
As the majority of lesions are small and nonfunctional, most adenomas are incidental findings.
Hyperfunctional adenomas also occur and are responsible for important endocrine disorders such as Cushing syndrome and Conn syndrome.
Lipid-rich adenomas (approximately 80% of adenomas) are easily identified at both CT and MR imaging.

Adrenal adenomas appear as small (<3 cm), well-defined homogeneous masses that are typically hypoattenuating relative to the liver.
At an attenuation value of less than 0 HU at unenhanced CT, the diagnosis of an adenoma can be made with 100% confidence; however, this threshold has only 47% sensitivity.
At cutoff of 18 HU, a diagnosis of adenoma was made with 100% specificity and 85% sensitivity, compared to the specificity:sensitivity ratio of 68%:100% with a more conservative cutoff of 10 HU.
A rational approach advocated by some authorities is to choose the CT number threshold on the basis of the patient’s risk for metastatic disease. For example, a threshold of 10 HU could be applied to older patients or to those with known primary malignancies. A threshold of 18 HU could be applied to younger patients without underlying cancer.

Lipid-poor adenomas are more difficult to diagnose because the CT numbers increase and approach those of soft tissue.
Contrast-enhanced imaging with 10-minute-delayed CT scans may be helpful in these cases.
- By using a threshold of 30 HU, the sensitivity and specificity for delayed contrast-enhanced CT in the characterization of benign disease are 80% and 100%, respectively.
- A relative percentage washout of more than 50% in the delayed study represents a sensitivity and specificity of 98% and 100%, respectively, for the detection of adenoma.

Chemical shift MR imaging can be used for further characterization when CT results are indeterminate.
Because of the high sensitivity of chemical shift MR imaging to minute amounts of intravoxel fat, MR imaging demonstrates signal intensity loss on opposed-phase images in the majority of adenomas, with a sensitivity of 89% for lesions with an attenuation of 10–30 HU and 100% for lesions with an attenuation of 10–20 HU with a maintained specificity of 100%.