What imaging study should be ordered to evaluate for adrenal tumors?

Imaging for Adrenal Tumors

Order a non-contrast CT scan of the abdomen as your first-line imaging study to evaluate for adrenal tumors. 1

Why Non-Contrast CT is First-Line

Non-contrast CT (NCCT) is explicitly recommended by endocrine clinical guidelines as the initial imaging test for adrenal lesions 1. This modality offers several practical advantages:

• Wider availability and shorter scan times compared to MRI
• Lower cost and better patient tolerability
• Critical diagnostic capability: Measures attenuation in Hounsfield Units (HU), which is the key to characterizing lesions

The 10 HU Rule

The non-contrast CT attenuation threshold of ≤10 HU is your primary decision point:

• ≤10 HU: Diagnostic of benign lipid-rich adenoma (98% specificity, 71% sensitivity) 1
• >10 HU: Requires further characterization with second-line imaging
• Important caveat: 100% of malignant lesions have attenuation >10 HU 1

For homogeneous lesions with attenuation between 11-20 HU, >90% are still benign 1, though these warrant closer evaluation.

When to Use Second-Line Imaging

If the non-contrast CT shows attenuation >10 HU and the lesion remains indeterminate, proceed to either:

1. Dedicated adrenal protocol CT (3-phase: non-contrast, venous phase at 60-70 seconds, delayed phase at 15 minutes) to assess contrast washout characteristics 1

   • Relative washout >40% or absolute washout >60% suggests adenoma
   • Critical limitation: ~1/3 of pheochromocytomas may show adenoma-like washout, and ~1/3 of adenomas don't washout in the typical range 2

2. Chemical shift MRI to detect intracellular lipid 1, 2

   • Signal loss on opposed-phase vs in-phase images indicates lipid-rich adenoma
   • No radiation exposure
   • Longer scan times and less availability than CT

PET Imaging

18F-FDG PET/CT is second-line and not routinely performed for adrenal lesions 1. Reserve this for specific clinical scenarios requiring functional imaging.

Clinical Context Matters

Do not order routine abdominal CT protocols (single venous phase) for characterizing a known adrenal lesion—these are inadequate 1. You need the non-contrast phase to measure HU values.

Size Considerations

While imaging characterizes the lesion, size influences malignancy risk:

• <3 cm: Almost all benign
• >6 cm: High risk of malignancy 3
• >4 cm with inhomogeneous appearance or HU >20: Surgery usually indicated 4

Common Pitfalls to Avoid

1. Never skip pheochromocytoma screening before any intervention—this is biochemical, not imaging-based, but critical for patient safety
2. Don't rely on contrast-enhanced CT alone without the non-contrast phase—you'll miss the diagnostic HU measurement
3. Don't biopsy suspected adrenocortical carcinoma—risk of tumor seeding 2
4. Biopsy is only appropriate when confirming metastatic disease from a known extra-adrenal malignancy would change management 2

Algorithmic Approach

1. Start: Order non-contrast CT abdomen
2. If HU ≤10 and homogeneous: Benign adenoma confirmed—proceed to hormonal evaluation only
3. If HU >10: Order either adrenal protocol CT with washout or chemical shift MRI
4. If still indeterminate after second-line imaging: Multidisciplinary discussion for surgical consideration, especially if >4 cm

This imaging strategy must be paired with comprehensive hormonal evaluation (dexamethasone suppression test, plasma/urinary metanephrines, and aldosterone screening if hypertensive), but the imaging pathway begins and often ends with non-contrast CT 1, 4.