What scan is used to determine if an adrenal nodule is benign?

Determining if an Adrenal Nodule is Benign: Optimal Imaging Approach

Non-contrast CT is the first-line imaging test to characterize adrenal masses, with lesions measuring <10 Hounsfield Units (HU) likely being benign adenomas. 1, 2

Initial Imaging Evaluation

• Non-contrast CT is the most appropriate initial scan for adrenal nodule characterization, offering high specificity for differentiating benign adenomas from malignant lesions 2
• Adrenal adenomas contain varying degrees of lipid, which lowers their attenuation coefficient on non-contrast CT, typically measuring <10 HU 2
• 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

Second-Line Imaging for Indeterminate Nodules

• 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:
  • Adenomas typically demonstrate >60% absolute washout at 15 minutes post-contrast 2
  • This technique has shown sensitivity >95% and specificity >97% for adenomas 2
• 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 2
  • Homogeneous signal intensity drop on MRI is diagnostic of lipid-rich adenoma 2
  • MRI is particularly helpful when non-enhanced CT is equivocal 2

Limitations and Pitfalls

• Washout CT has limitations that clinicians should be aware of:
  • Approximately 1/3 of pheochromocytomas may washout in the characteristic range of an adenoma 2
  • 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, 3
• Recent multiinstitutional research suggests washout CT has suboptimal performance for characterizing nodules as benign in patients without known malignancy 3
• Heterogeneous signal intensity drop on MRI is a controversial finding as minute amounts of microscopic fat have been identified in pheochromocytomas, adrenal cortical carcinomas, and some metastases 2

Follow-up Imaging

• 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

Role of Biopsy

• Adrenal biopsy should not be routinely performed in the work-up of adrenal incidentalomas 4
• Fine-needle aspiration biopsy should be reserved for cases where non-invasive techniques are equivocal 1
• Biochemical exclusion of pheochromocytoma is mandatory before any biopsy due to risk of hypertensive crisis 1, 4

Special Considerations

• Some adrenal lesions have characteristic features that allow definitive diagnosis on imaging alone:
  • Myelolipomas and cysts are readily identified by CT by their imaging features 2, 5
  • Size >4 cm increases suspicion for malignancy, particularly with irregular margins or heterogeneous appearance 2
• For patients with known malignancy, PET scanning may be considered if CT and MRI are inconclusive 2

By following this imaging algorithm, clinicians can effectively differentiate benign from malignant adrenal nodules in most cases, minimizing unnecessary procedures while ensuring appropriate management of potentially malignant lesions.