What imaging study is recommended for a mass on the adrenal gland?

Imaging for Adrenal Mass Evaluation

For evaluation of an adrenal mass, non-contrast CT of the abdomen is the first-line imaging modality, with an appropriateness rating of 8/9 according to the American College of Radiology guidelines. 1

Initial Imaging Approach

Non-contrast CT

• First-line imaging modality for adrenal mass evaluation
• Highly specific for differentiating adenomas from malignant lesions
• Adenomas typically appear as homogeneous, well-circumscribed masses
• Key diagnostic criterion: Hounsfield Units (HU) < 10 indicates benign adenoma with 0% risk of adrenocortical carcinoma 2
• Sensitivity can be improved through histographic analysis if at least 5% of pixels are negative 1

When Non-contrast CT is Indeterminate (HU > 10)

If the initial non-contrast CT shows a mass with HU > 10 or if the mass is discovered on a contrast-enhanced CT, additional imaging is needed:

1. Delayed Enhanced CT (Washout Protocol)

   • Appropriateness rating: 8/9 1
   • Adenomas show faster contrast washout compared to malignant lesions

   • 60% washout at 15 minutes suggests benign lesion 2

   • Sensitivity >95%, specificity >97% 1

2. Chemical Shift MRI

   • Appropriateness rating: 8/9 1
   • Particularly useful when lesion is identified only on contrast-enhanced CT
   • Based on detecting intracytoplasmic lipid content in adenomas
   • Signal intensity loss in opposed-phase images indicates benign adenoma
   • High sensitivity (96%) and specificity (100%) for differentiating benign from malignant lesions 2
   • Can correctly characterize up to 89% of lesions with CT densities between 10-30 HU 1

Imaging Based on Clinical Context

No History of Malignancy, Mass < 3 cm

• Non-contrast CT or chemical shift MRI is usually sufficient
• Follow-up imaging in 6-12 months to ensure stability 1

No History of Malignancy, Mass 3-5 cm

• More aggressive evaluation recommended
• Consider follow-up imaging in 3-6 months
• Higher risk of malignancy requires more thorough evaluation 1

History of Malignancy

• More aggressive evaluation needed due to higher risk of metastasis
• Consider FDG-PET (appropriateness rating: 6/9) if CT and MRI are inconclusive
• Adrenal biopsy may be necessary (appropriateness rating: 8/9) 1

Additional Imaging Considerations

FDG-PET

• Useful for detecting metabolically active lesions, particularly metastases
• Especially valuable for evaluating adrenal masses in patients with lung cancer, colon cancer, lymphoma, and neuroendocrine tumors
• Specific uptake values >4 typically indicate metastatic disease 1

Specialized Nuclear Medicine Studies

• Iodocholesterol scan: Limited utility (appropriateness rating: 2-3/9), only for functionally active lesions 1
• MIBG scan: Only indicated for suspected pheochromocytoma (appropriateness rating: 2-3/9) 1

Important Caveats

• Masses ≥4 cm should be considered for surgical removal due to increased risk of malignancy, regardless of imaging characteristics 2
• All adrenal masses require hormonal evaluation regardless of imaging findings 2
• Avoid adrenal biopsy without first excluding pheochromocytoma due to risk of hypertensive crisis 1
• Ultrasound and plain radiographs have limited utility (appropriateness rating: 2/9) 1

Size-Based Recommendations

• <3 cm with benign features: Follow-up imaging in 6-12 months
• 3-5 cm: Consider follow-up in 3-6 months or surgical evaluation

• 5 cm: Surgical removal recommended due to high risk of malignancy 1, 2

Remember that while imaging is crucial for characterization, hormonal evaluation is equally important for complete assessment of adrenal masses.