Is a CT (Computed Tomography) scan with contrast (CT with) enough for the evaluation of an adrenal incidentaloma, or is a CT scan with and without contrast (CT w/wo) recommended?

CT Protocol for Adrenal Incidentaloma Evaluation

For adrenal incidentaloma evaluation, non-contrast CT is the first-line imaging modality, and if indeterminate (>10 HU), a delayed enhancement CT (washout CT) or chemical shift MRI should be performed as second-line imaging. 1

Initial Imaging Approach

• Non-contrast CT is the recommended first-line imaging modality for adrenal incidentaloma characterization, with homogeneous, well-circumscribed lesions <10 Hounsfield Units (HU) considered benign 1
• If the adrenal mass shows >10 HU on non-contrast CT, it is considered indeterminate and requires further imaging evaluation 1, 2
• The American College of Radiology assigns an appropriateness rating of 8/9 (highly appropriate) for both non-contrast CT and delayed enhancement CT in the evaluation of adrenal incidentalomas 1

Second-Line Imaging Options

• For indeterminate adrenal masses on non-contrast CT, two options exist for second-line imaging:

Option 1: Delayed Enhancement CT (Washout CT)

• Delayed enhancement CT evaluates the washout characteristics of adrenal masses after contrast administration 1
• Adenomas typically demonstrate rapid washout of contrast (>60% absolute washout or >40% relative washout at 15 minutes) 1
• Studies show excellent sensitivity (>95%) and specificity (>97%) for adenoma detection with this technique 1

Option 2: Chemical Shift MRI

• Chemical shift MRI exploits the different frequency of protons in water and fat to detect microscopic fat 1
• Homogeneous signal intensity drop on MRI is diagnostic of lipid-rich adenoma 1
• The American College of Radiology assigns an appropriateness rating of 8/9 for chemical shift MRI in adrenal incidentaloma evaluation 1

Important Limitations and Pitfalls

• Approximately 1/3 of pheochromocytomas may demonstrate washout in the characteristic range of an adenoma, potentially leading to false negatives 1, 2
• About 1/3 of adrenal adenomas do not washout in the adenoma range 1
• Malignant masses can sometimes washout in the adenoma range, resulting in adrenal cortical carcinoma or hypervascular metastases being mistaken for adenomas 1
• 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 1

Decision Algorithm

1. Start with non-contrast CT:

   • If <10 HU and homogeneous: Benign adenoma, no further imaging needed 1
   • If >10 HU or heterogeneous: Proceed to second-line imaging 1

2. Choose second-line imaging:

   • Delayed enhancement CT (washout CT) OR
   • Chemical shift MRI 1

3. Interpret second-line imaging:

   • If rapid washout on CT or signal drop on MRI: Likely benign adenoma 1
   • If poor washout or no signal drop: Consider functional testing and possible surgical evaluation 1

Special Considerations

• All patients with adrenal incidentalomas should undergo hormonal evaluation regardless of imaging characteristics, as imaging cannot reliably predict functionality 1, 3
• Biochemical testing (plasma or 24-hour urinary metanephrines) must be performed before any invasive procedure, including biopsy, to exclude pheochromocytoma and avoid hypertensive crisis 2
• Adrenal mass biopsy is rarely indicated and should only be considered when diagnosis of metastatic disease from an extra-adrenal malignancy would be of value 1

Remember that while CT with contrast alone can provide valuable information, the most accurate characterization of adrenal incidentalomas comes from the combination of non-contrast CT followed by appropriate second-line imaging when indicated.