CT Scan Ordering Protocol for Suspected Adrenal Tumor
Order a non-contrast CT of the abdomen as your initial imaging study—this is the first-line test recommended by the American College of Radiology with an appropriateness rating of 8/9. 1, 2, 3
Initial Imaging: Non-Contrast CT
- Non-contrast CT is essential because it measures the attenuation (density) of the adrenal mass in Hounsfield Units (HU), which is the most specific initial test for characterizing adrenal lesions 1, 2, 3
- Lesions measuring ≤10 HU are diagnostic of benign lipid-rich adenomas and require no further imaging characterization 1, 2, 3, 4
- This threshold of 10 HU has excellent specificity for differentiating benign adenomas from malignant lesions 1, 2
When to Order Additional CT Imaging
If the non-contrast CT shows >10 HU or the mass was discovered incidentally on a contrast-enhanced study, order a delayed enhancement (washout) CT protocol: 1, 2, 3
- Obtain contrast-enhanced images followed by delayed images at 15 minutes post-contrast 1, 3
- Adenomas demonstrate >60% absolute washout (or >40% relative washout) at 15 minutes, indicating benignity 1, 3
- The American College of Radiology assigns delayed enhancement CT an appropriateness rating of 8/9 for indeterminate masses 1, 3
Critical Pitfall to Avoid
- Approximately one-third of pheochromocytomas can show washout patterns mimicking adenomas, and some malignant masses can also demonstrate adenoma-like washout 2, 3
- You must biochemically exclude pheochromocytoma before any biopsy by ordering plasma or 24-hour urinary metanephrines to avoid potentially fatal hypertensive crisis 1, 3
Alternative Second-Line Imaging: Chemical Shift MRI
If the patient cannot receive contrast or if the adrenal mass was discovered on MRI, order chemical shift MRI instead of washout CT: 1, 2, 3
- Chemical shift MRI has an appropriateness rating of 8/9 from the American College of Radiology 1, 3
- Homogeneous signal intensity drop on opposed-phase images is diagnostic of lipid-rich adenoma 2, 3
- This technique exploits microscopic fat content to differentiate benign from malignant lesions 2, 3
Size-Based Imaging Considerations
The size of the mass influences your imaging approach:
- Masses <3 cm: Most are benign; if imaging characteristics are benign (≤10 HU), order follow-up CT or MRI at 6-12 months to document stability 1
- Masses 4-6 cm with benign features: Order repeat imaging at 3-6 months initially, then 6-12 months if stable 1, 5
- Masses >6 cm or those with aggressive features (irregular margins, inhomogeneous appearance): These have high malignancy risk and require CT chest/abdomen/pelvis to evaluate for metastases 1, 6
When to Consider Biopsy
Do not routinely order adrenal biopsy for incidentalomas—reserve it for specific situations: 1, 2
- The American College of Radiology assigns biopsy an appropriateness rating of only 4/9 for masses without cancer history 1
- Order image-guided needle biopsy only if: the patient has a known extra-adrenal malignancy with suspected adrenal metastasis as the only site of disease, AND non-invasive imaging (CT/MRI) remains equivocal 1, 2
- Biopsy has an appropriateness rating of 8/9 in patients with known malignancy 1
Role of PET Imaging
Consider ordering FDG-PET scan in highly selected cases: 1, 2
- PET has an appropriateness rating of 5-6/9 and should be reserved for when CT and MRI are inconclusive 1
- Most useful for lung cancer, colon cancer, lymphoma, and neuroendocrine tumors 1
- False negatives occur with renal cell carcinoma metastases, so PET is not the method of choice for this primary tumor 1
Imaging Features Suggesting Malignancy
Order expedited imaging workup if the mass demonstrates: 1, 6