Is a 12x17 mm adrenal nodule in an adult likely to be malignant?

Is a 12x17 mm Adrenal Nodule Malignant?

A 12x17 mm (1.7 cm) adrenal nodule is highly unlikely to be malignant, with approximately 99% probability of being benign in patients without a history of cancer. 1, 2

Size-Based Risk Assessment

• **In patients without known malignancy, nodules <3 cm have an 87% probability of being benign**, and importantly, all malignant lesions in this population were >5 cm in published series. 1, 2

• Your nodule at 1.7 cm falls well below the 3 cm threshold where malignancy risk begins to increase meaningfully. 1

• The prevalence of malignancy in nodules <4 cm without cancer history is only 0.3% (95% CI: 0.0-1.7%), based on a large multi-institutional study. 3

Critical Next Steps: Imaging Characterization

You must obtain a dedicated non-contrast CT scan to measure the nodule's density in Hounsfield Units (HU) as the essential first diagnostic step. 4, 5

Interpretation Algorithm:

• If unenhanced CT shows <10 HU: The nodule is definitively a benign lipid-rich adenoma requiring no further imaging workup. 4, 5

• If unenhanced CT shows >10 HU: Proceed immediately to either:

  • Delayed contrast-enhanced CT with 15-minute washout imaging (if >60% washout = likely benign), OR
  • Chemical-shift MRI to assess for microscopic fat content. 1, 4, 5

Mandatory Hormonal Evaluation

Regardless of imaging characteristics, you must perform endocrinologic screening because approximately 5% of radiologically benign incidentalomas have subclinical hormone production requiring treatment. 1, 5

Required Tests:

• Plasma aldosterone and renin activity (to exclude hyperaldosteronism). 1
• Serum ACTH, cortisol, and DHEA-S with overnight 1 mg dexamethasone suppression test (to exclude Cushing syndrome). 1
• Fractionated plasma-free metanephrines (to exclude pheochromocytoma). 1

Follow-Up Strategy for Benign-Appearing Nodules

If imaging confirms a benign adenoma (<10 HU), repeat imaging in 6-12 months is reasonable to document stability, though extensive follow-up is not mandatory for nodules this small. 1, 5

• **Benign adenomas grow at <3 mm/year**, while all malignant nodules grow at >5 mm/year. 6
• If the nodule enlarges by >1 cm in 1 year, consider adrenalectomy for suspected malignancy. 1

Critical Pitfall to Avoid

Never proceed directly to adrenal biopsy without first excluding pheochromocytoma through biochemical testing, as biopsy of an undiagnosed pheochromocytoma can cause potentially fatal hypertensive crisis. 1, 5

Special Consideration: Cancer History

If you have a history of malignancy (particularly lung, colon, melanoma, or renal cell carcinoma), the risk profile changes—even small nodules become more suspicious, though your 1.7 cm nodule would still have only 13-21% probability of being metastatic. 1

• In cancer patients, FDG-PET imaging may be useful, as metastases typically show SUV >4 while benign lesions show SUV <4. 1, 5