What is the recommended CT protocol for an adult patient with an incidentally discovered adrenal mass on a lung CT scan?

CT Protocol for Incidentally Discovered Adrenal Mass

Start with a dedicated non-contrast CT of the abdomen to measure the adrenal mass density in Hounsfield Units (HU), and if the mass measures >10 HU, immediately proceed to delayed contrast-enhanced CT (washout protocol) or chemical shift MRI for further characterization. 1, 2

Initial Imaging: Non-Contrast CT First

• The non-contrast CT is the essential first step and receives the highest appropriateness rating (8 out of 9) from the American College of Radiology for evaluating incidentally discovered adrenal masses. 3, 1
• If the mass measures <10 HU and appears homogeneous and well-circumscribed, it is definitively a benign lipid-rich adenoma requiring no further imaging workup. 1, 2
• This single measurement can immediately characterize approximately 50-70% of adrenal incidentalomas as benign adenomas, avoiding unnecessary additional imaging. 4

When to Add Contrast: The "Adrenal Protocol"

Yes, request an "adrenal protocol" CT if the non-contrast study shows >10 HU. This consists of:

• Delayed enhancement CT (washout protocol) with imaging at 15 minutes post-contrast administration. 3, 2
• Adenomas demonstrate rapid washout of contrast (>60% absolute washout or >40% relative washout at 15 minutes), achieving sensitivity >95% and specificity >97%. 3
• The American College of Radiology assigns delayed enhancement CT an appropriateness rating of 8 out of 9, equal to non-contrast CT. 3, 2

Alternative Second-Line Imaging

• Chemical shift MRI is an equally appropriate alternative (appropriateness rating 8 out of 9) if the non-contrast CT is indeterminate. 3, 2
• Homogeneous signal intensity drop on MRI is diagnostic of lipid-rich adenoma. 3, 2
• Choose MRI over washout CT in younger patients to minimize radiation exposure, or when contrast is contraindicated. 4

Critical Pitfalls to Avoid

• Do not skip the non-contrast phase. If the adrenal mass was discovered on a contrast-enhanced lung CT, you still need a dedicated non-contrast study to measure HU accurately. 1, 2
• Be aware that approximately 1/3 of pheochromocytomas may washout in the characteristic range of an adenoma, potentially leading to false negatives on washout CT. 3, 2
• Approximately 1/3 of adrenal adenomas do not washout in the adenoma range, and malignant masses can sometimes washout similarly to adenomas. 3, 4
• Never proceed to biopsy without first excluding pheochromocytoma biochemically, as biopsy carries risk of hypertensive crisis and tumor seeding. 3, 1

Size-Based Considerations

• Masses <3 cm are usually benign, but still require the non-contrast CT characterization protocol. 3, 1
• Masses 3-5 cm require second-line imaging (washout CT or chemical shift MRI) regardless of HU measurement. 1
• Masses ≥4 cm require repeat imaging in 6-12 months even if radiologically benign (<10 HU). 3, 1
• Masses >5 cm should be removed due to higher malignancy risk. 1

Practical Ordering Instructions

Order this way:

1. "CT abdomen without contrast, dedicated adrenal protocol with thin cuts through adrenals"
2. If HU >10: "CT abdomen with IV contrast and 15-minute delayed images for washout calculation" OR "MRI abdomen with chemical shift imaging"

Do not order: "CT abdomen with and without contrast" as a single study, because the timing of the delayed images (15 minutes) is specific and different from routine portal venous phase imaging. 3, 2

Concurrent Hormonal Evaluation

• Initiate hormonal workup simultaneously regardless of imaging characteristics, as approximately 5% of radiologically benign incidentalomas have subclinical hormone production requiring treatment. 1
• Biochemical testing for pheochromocytoma is mandatory before any invasive procedure. 3, 2