What imaging studies should be performed to further evaluate an incidentally discovered adrenal adenoma?

Imaging Evaluation of Incidentally Discovered Adrenal Adenoma

For an incidentally discovered adrenal adenoma, obtain a non-contrast CT to measure Hounsfield Units (HU) as the essential first step; if HU ≤10, the lesion is definitively a benign lipid-rich adenoma requiring no further imaging, but if HU >10, proceed to second-line imaging with either washout CT or chemical shift MRI. 1, 2

Initial Imaging: Non-Contrast CT

• Non-contrast CT measuring attenuation in Hounsfield Units is the mandatory first imaging test that determines whether the mass is benign or requires additional characterization 1, 2
• A lesion measuring ≤10 HU is definitively a benign lipid-rich adenoma with 100% specificity and requires no further imaging workup 1, 2, 3
• Lesions measuring >10 HU are indeterminate and require second-line imaging to differentiate lipid-poor adenomas from malignant lesions 1, 2
• The 10 HU threshold provides excellent specificity (approaching 100%) with no false-positive results, though sensitivity is only 48-56% 1

Second-Line Imaging for Indeterminate Masses (HU >10)

Option 1: Contrast-Enhanced Washout CT

• Perform dynamic contrast-enhanced CT followed by delayed images at 10-15 minutes to calculate washout characteristics 1, 2
• Calculate absolute percentage washout = [(enhanced HU - delayed HU) / (enhanced HU - unenhanced HU)] × 100%; a value >60% indicates benign adenoma 1, 4
• Calculate relative percentage washout = [(enhanced HU - delayed HU) / enhanced HU] × 100%; a value >40% indicates benign adenoma 1, 4
• This protocol achieves approximately 98% accuracy for characterizing adrenal masses 2

Critical limitation: Approximately one-third of benign adenomas fail to washout in the typical adenoma range, and conversely, some malignant masses (including adrenocortical carcinoma and hypervascular metastases) can demonstrate washout values in the adenoma range 1, 5

Option 2: Chemical Shift MRI

• Out-of-phase MRI showing signal loss compared to in-phase images identifies microscopic fat characteristic of benign adenomas 1, 2
• Chemical shift MRI correctly characterizes approximately 89% of lesions with non-contrast CT attenuation between 10-30 HU 1, 2
• MRI offers higher sensitivity and specificity than CT alone for indeterminate lesions and avoids additional radiation exposure 1, 2
• Homogeneous signal intensity drop is diagnostic of lipid-rich adenoma; heterogeneous signal drop is more controversial as minute amounts of microscopic fat can occur in pheochromocytomas and adrenocortical carcinomas 1

Size-Based Risk Stratification

• Masses <3 cm are most often benign and extensive workup is generally not justified in patients without known extra-adrenal malignancy 1
• Masses 3-5 cm warrant either follow-up imaging or additional characterization depending on imaging characteristics 1
• Masses >5 cm should be surgically removed due to substantially higher risk of malignancy 1

Advanced Imaging: Limited Role

FDG-PET Imaging

• FDG-PET is NOT recommended for initial characterization of adrenal incidentalomas 2, 5
• Reserve FDG-PET only for lesions that remain indeterminate after both CT and MRI have been performed 1, 2
• Some malignancies (including renal cell carcinoma metastases) may not be PET-avid, limiting sensitivity 1

Imaging Modalities to Avoid

• Plain radiography and abdominal ultrasound have very limited diagnostic value for adrenal lesions and should not be used 1, 2

Role of Biopsy: Rarely Indicated

• Adrenal biopsy should NOT be routinely performed due to risks of tumor seeding, hemorrhage, and potential hypertensive crisis 1, 2, 5
• Biopsy may be considered only when non-invasive imaging remains indeterminate AND the patient has a known extra-adrenal malignancy where confirming metastatic disease would alter management 1
• Prior to any biopsy, pheochromocytoma must be biochemically excluded with plasma or 24-hour urinary metanephrines to prevent life-threatening hypertensive crisis 1, 2, 5
• Suspected adrenocortical carcinoma should not be biopsied due to risk of needle tract seeding 1

Critical Pitfalls to Avoid

• Do not rely on washout CT alone as approximately one-third of pheochromocytomas washout in the characteristic range of adenomas, and one-third of adenomas do not washout in the adenoma range 1, 5
• Never proceed with biopsy before excluding pheochromocytoma biochemically, as undiagnosed pheochromocytoma can cause fatal hypertensive crisis during the procedure 1, 2, 5
• Avoid unnecessary repeated imaging for small masses (<4 cm) with confirmed benign characteristics (≤10 HU, homogeneous), as this increases radiation exposure, patient anxiety, and healthcare costs without clinical benefit 2, 5