What is the follow-up for low-density bilateral adrenal nodules with Hounsfield units (HU) less than -10?

Follow-up for Low Density Bilateral Adrenal Nodules Less Than -10 Hounsfield Units

For bilateral adrenal nodules with Hounsfield units (HU) less than -10, no further imaging follow-up or functional testing is required as these represent benign lipid-rich adenomas with extremely low risk of malignancy or hormonal hyperfunction. 1

Radiological Characteristics and Risk Assessment

• Adrenal nodules with HU less than 10 on unenhanced CT are considered benign regardless of size, with those less than -10 HU representing lipid-rich adenomas with even higher confidence of benignity 1
• The threshold value of 10 HU has been established as a reliable means of distinguishing benign from malignant adrenal tumors, with values below this threshold strongly indicating benign lesions 1
• Low-density adrenal nodules (<10 HU) have been shown to maintain their benign characteristics over time, with 97% of such masses continuing to demonstrate low attenuation on follow-up imaging 2

Management Algorithm for Bilateral Adrenal Nodules <-10 HU

Initial Approach

• For nodules <4 cm with HU <10 (especially those <-10 HU), no further imaging follow-up is recommended as per current guidelines 1
• Each nodule in bilateral adrenal incidentalomas should be separately characterized, but the same follow-up principles apply to each nodule 1

Special Considerations for Bilateral Nodules

• In bilateral adrenal incidentalomas, consider measuring serum 17-hydroxyprogesterone to exclude congenital adrenal hyperplasia 1
• Assessment for adrenal insufficiency should be considered in cases of bilateral infiltrative disease, though this is extremely unlikely with nodules <-10 HU 1

Evidence Supporting No Follow-up

• A 5-year prospective follow-up study of lipid-rich adrenal incidentalomas (<10 HU) showed no clinically relevant tumor growth or development of hormonal hypersecretion 2
• The 2023 Canadian Urological Association/American Urological Association guidelines explicitly state that "patients with benign non-functional adenomas <4 cm, myelolipomas, and other small masses containing macroscopic fat detected on the initial work-up for an adrenal incidentaloma do not require further follow-up imaging or functional testing" 1
• The prevalence of malignancy in adrenal nodules <4 cm with unenhanced attenuation <10 HU is extremely low (0% in recent studies) 3

Exceptions to Consider

• If either nodule is ≥4 cm despite having HU <-10, a single repeat imaging in 6-12 months may be considered due to the slightly higher risk associated with larger size 1, 3
• If there are any signs of hormonal excess despite the benign imaging appearance, appropriate hormonal evaluation should be performed 4
• Growth >5 mm/year on any follow-up imaging would warrant consideration for adrenalectomy after repeating functional work-up 1

Common Pitfalls to Avoid

• Avoid unnecessary follow-up imaging for nodules with clearly benign characteristics (HU <-10), as this leads to increased healthcare costs, radiation exposure, and patient anxiety 3, 5
• Avoid assuming all bilateral adrenal nodules represent metastatic disease; bilateral adenomas are common, especially in older patients 1
• Do not confuse the management of lipid-rich adenomas (HU <10) with indeterminate adrenal masses (HU >10), which require more extensive follow-up 1