What is the next step in managing a lesion with indeterminate CT scan results, showing Hounsfield units (HU) of 34, 90, and 60 on the 3 phases, and absolute and relative washout values of 60% and 40%, respectively?

Management of Adrenal Lesion with Indeterminate Washout Characteristics

For an adrenal lesion with indeterminate washout values (60% absolute, 40% relative), the next step is to obtain additional imaging with chemical shift MRI or consider PET scanning if malignancy risk is elevated, followed by close interval imaging surveillance at 3-6 months if these studies remain indeterminate. 1

Understanding the Indeterminate Results

Your lesion demonstrates washout values that fall exactly at the threshold cutoffs used to distinguish benign from malignant lesions:

• Absolute washout of 60%: This is at the exact threshold where ≥60% suggests benign pathology 2
• Relative washout of 40%: This is also at the exact threshold where ≥40% suggests benign pathology 2
• Unenhanced attenuation of 34 HU: This is significantly above the 10 HU threshold that would definitively indicate a benign lipid-rich adenoma 1

The Canadian Urological Association, American Association of Clinical Endocrinologists/American Association of Endocrine Surgeons, American College of Radiology, and Korean Endocrine Society all recommend proceeding with enhanced CT with washout for equivocal initial imaging, which you have already completed 2. Since these results remain indeterminate, additional steps are warranted.

Recommended Next Steps

Primary Recommendation: Chemical Shift MRI

Chemical shift MRI should be obtained as the next imaging modality to detect microscopic fat content that may not be apparent on CT. 1 This technique exploits the different frequency of protons in water and fat, and a homogeneous signal intensity drop on MRI is diagnostic of a lipid-rich adenoma even when CT attenuation is elevated. 1

• Chemical shift MRI is recommended by the European Society of Endocrinology as second-line imaging when CT with washout is equivocal 2
• This modality can identify adenomas that are lipid-poor on CT but still contain microscopic fat detectable by MRI 1

Alternative: PET Scanning

PET scanning may be considered if you have a known history of malignancy or if clinical suspicion for malignancy is high based on lesion size or patient characteristics. 1

• PET is particularly useful when non-invasive techniques remain equivocal 1
• However, approximately one-third of pheochromocytomas may demonstrate washout patterns similar to adenomas, which is a critical pitfall 1

Mandatory Concurrent Evaluation

Hormonal Testing

All adrenal incidentalomas require complete hormonal evaluation regardless of imaging characteristics: 2, 3

• Low-dose dexamethasone suppression test (1 mg overnight) or 2-3 midnight salivary cortisol measurements to exclude subclinical Cushing syndrome 2, 3
• Plasma-free and/or urinary fractionated metanephrines to rule out pheochromocytoma 2
• Aldosterone-to-renin ratio only if hypertension or unexplained hypokalemia is present 2

Critical pitfall: Biochemical exclusion of pheochromocytoma is mandatory before any biopsy procedure due to risk of hypertensive crisis. 1

Surveillance Strategy if Additional Imaging Remains Indeterminate

If chemical shift MRI and/or PET scanning do not provide definitive characterization, implement close interval surveillance: 1, 3

• Initial follow-up imaging at 3-6 months to assess for growth or changes in imaging characteristics 1
• Subsequent imaging at 9-12 months and 18-24 months if the lesion remains stable 3
• Growth of >5 mm/year warrants surgical consideration after repeating functional workup 3, 4
• Growth of <3 mm/year allows discontinuation of surveillance 3

Size Considerations

The size of your lesion is not specified in the question, but this is critical for management decisions:

• Lesions ≥4 cm have increased suspicion for malignancy and warrant more aggressive evaluation, potentially including surgical resection even if radiologically benign 1, 4
• Lesions <4 cm that are confirmed benign and non-functional can be safely observed 3, 4

When to Consider Biopsy

Adrenal biopsy should NOT be routinely performed and should be reserved only for cases where: 1

• Non-invasive imaging techniques remain equivocal after exhausting all modalities
• A specific benign diagnosis requiring medical treatment is suspected
• The patient has known extra-adrenal malignancy and metastasis needs to be confirmed

Always exclude pheochromocytoma biochemically before any biopsy. 1

Special Considerations

Approximately one-third of adrenal adenomas do not demonstrate typical washout patterns in the adenoma range, and conversely, some malignant masses can demonstrate washout similar to adenomas. 1 This underscores why your indeterminate results require additional evaluation rather than assuming benignity based solely on borderline washout values.

The accuracy of washout analysis is preserved even in cortisol-secreting adenomas, though these lesions are more likely to have higher unenhanced attenuation values due to lipid-poor content. 5