Characterization of Adrenal Nodules using Imaging Techniques

Initial Imaging Evaluation

The American College of Radiology recommends non-contrast CT as the first-line imaging test to characterize adrenal masses, with lesions measuring <10 Hounsfield Units (HU) likely being benign adenomas 1, 2
Adrenal adenomas contain varying degrees of lipid, which lowers their attenuation coefficient on non-contrast CT, typically measuring <10 HU 3, 4
A threshold value of 10 HU is generally accepted as the cutoff value for identifying benign adenomas, with lesions measuring <10 HU on non-contrast CT considered benign 2, 1

For adrenal masses that are indeterminate on non-contrast CT (>10 HU), second-line imaging with either washout CT or chemical shift MRI is recommended 2, 1
Washout CT involves contrast enhancement followed by delayed imaging to assess contrast washout patterns, with adenomas typically demonstrating >60% absolute washout at 15 minutes post-contrast 3, 5
Chemical shift MRI is an alternative second-line imaging option that exploits the different frequency of protons in water and fat to detect microscopic fat, with homogeneous signal intensity drop on MRI being diagnostic of lipid-rich adenoma 2, 4

Approximately 1/3 of pheochromocytomas may washout in the characteristic range of an adenoma, and about 1/3 of adrenal adenomas do not washout in the adenoma range 2
Malignant masses can sometimes demonstrate washout patterns similar to adenomas 2

For indeterminate lesions, follow-up CT or MRI at 3-6 months is appropriate to assess for growth or changes in imaging characteristics 1
Patients with non-functional adrenal lesions that are radiologically benign (<10 HU) but ≥4 cm should undergo repeat imaging in 6-12 months 2
No further imaging follow-up is required for patients with adrenal lesions that grow <3 mm/year on follow-up imaging 2

Adrenal biopsy should not be routinely performed in the work-up of adrenal incidentalomas, and fine-needle aspiration biopsy should be reserved for cases where non-invasive techniques are equivocal 6, 1
Biochemical exclusion of pheochromocytoma is mandatory before any biopsy due to risk of hypertensive crisis 1, 6

Some adrenal lesions have characteristic features that allow definitive diagnosis on imaging alone, such as myelolipomas and cysts, which are readily identified by CT by their imaging features 3
Size >4 cm increases suspicion for malignancy, particularly with irregular margins or heterogeneous appearance 5
For patients with known malignancy, PET scanning may be considered if CT and MRI are inconclusive 3, 4