What is the recommended CT protocol for an adrenal gland mass?

CT Protocol for Adrenal Gland Mass

For initial evaluation of an adrenal mass, obtain a non-contrast CT first—if the lesion measures ≤10 Hounsfield Units (HU), it is a benign adenoma and requires no further imaging; if >10 HU, proceed to delayed enhancement CT (washout protocol) at 15 minutes post-contrast. 1, 2, 3

Initial Imaging: Non-Contrast CT

• Non-contrast CT is the first-line imaging modality with an appropriateness rating of 8/9 from the American College of Radiology 1, 3
• Measure attenuation in Hounsfield Units (HU) using a region of interest (ROI) placed in the center of the lesion 2, 4
• Lesions measuring ≤10 HU are benign lipid-rich adenomas with 96-98% specificity—no further imaging is needed 1, 2, 4
• Lesions >10 HU are indeterminate and require additional characterization 1, 2

Second-Line Imaging: Delayed Enhancement CT (Washout Protocol)

When non-contrast CT shows >10 HU or when the mass is discovered on contrast-enhanced CT:

• Administer intravenous contrast and obtain portal venous phase images (60-90 seconds) 1, 5
• Obtain delayed images at 15 minutes post-contrast 1, 5
• Calculate absolute percentage washout: [(enhanced HU - delayed HU) / (enhanced HU - unenhanced HU)] × 100 1, 4
• Absolute washout ≥60% indicates benign adenoma with sensitivity >95% and specificity >97% 1, 4
• Adenomas demonstrate rapid contrast washout due to less "leaky" vasculature compared to malignant lesions 1

Alternative Second-Line Imaging: Chemical Shift MRI

• Chemical shift MRI is equally appropriate (rating 8/9) when CT is indeterminate 1, 3
• Particularly useful when lesion is identified only on contrast-enhanced CT and further characterization is required 1
• Detects microscopic fat content—homogeneous signal drop on out-of-phase images compared to in-phase images is diagnostic of lipid-rich adenoma 2, 3
• MRI may have better sensitivity than non-contrast CT alone, correctly characterizing 89% of lesions with CT density 10-30 HU 1

Size-Based Approach

For masses <3 cm without malignancy history:

• Non-contrast CT (rating 8/9) 1
• If indeterminate, delayed enhancement CT (rating 8/9) 1
• Follow-up CT or MRI at 6-12 months (rating 8/9) 1, 2

For masses 3-5 cm without malignancy history:

• Non-contrast CT (rating 8/9) 1
• If indeterminate, delayed enhancement CT (rating 8/9) 1
• Follow-up CT or MRI at 3-6 months (rating 8/9) 1
• Masses >5 cm should be surgically removed 1

Critical Pitfalls to Avoid

• Approximately 1/3 of pheochromocytomas may demonstrate washout in the adenoma range (≥60%), potentially causing false negatives 2, 3
• Some hypervascular metastases and adrenocortical carcinomas can show rapid washout, mimicking adenomas 2, 3
• Heterogeneous adrenal nodules have poor diagnostic performance with standard washout protocols regardless of ROI placement method, with sensitivity only 57-68% 6
• Masses >4 cm with irregular margins, heterogeneous appearance, or HU >10 on non-contrast CT raise suspicion for malignancy 1, 2

When to Consider Biopsy

• Biopsy should only be performed when CT and MRI are inconclusive (rating 4-6/9) 1, 7
• Biochemical exclusion of pheochromocytoma is mandatory before any biopsy—several deaths have been reported from biopsying undiagnosed pheochromocytomas 1, 7
• Complication rates are 8-12% including bleeding, pneumothorax, infection, and tumor seeding 1, 7
• Biopsy has 96-100% accuracy for malignant lesions when adequate tissue is obtained, but 4-19% of samples are insufficient 1

Follow-Up Imaging Strategy

• For radiologically benign lesions (≤10 HU) that are ≥4 cm: repeat imaging at 6-12 months 2
• For indeterminate lesions: follow-up at 3-6 months to assess for growth 1, 7
• No further imaging if growth is <3 mm/year 2
• Lesions deemed low-risk based on small size should be followed at 3-12 months, though slow-growing metastases can occur 1