Unenhanced CT Attenuation — First Branch Point
If unenhanced CT is available, attenuation is the primary decision driver:
| HU on Unenhanced CT | Interpretation | Recommendation |
|---|---|---|
| ≤10 HU | Lipid-rich adenoma (~98% specific) | No further imaging; biochemical evaluation only if clinically indicated |
| 11–30 HU | Indeterminate — may be lipid-poor adenoma | Adrenal protocol CT washout or chemical shift MRI |
| >30 HU | Indeterminate / concerning | Adrenal protocol CT washout; consider PET or biopsy if suspicious |
| No unenhanced CT available | Indeterminate | Adrenal protocol CT or chemical shift MRI |
Washout CT Criteria
Adrenal protocol CT uses 15-minute delayed imaging to assess contrast washout:
| Washout Measure | Formula | Threshold for Adenoma |
|---|---|---|
| Absolute percentage washout (APW) | (enhanced HU − 15-min HU) ÷ (enhanced HU − pre-contrast HU) × 100 | ≥60% |
| Relative percentage washout (RPW) | (enhanced HU − 15-min HU) ÷ enhanced HU × 100 | ≥40% (use when no pre-contrast available) |
If washout criteria are met → adenoma, no further imaging. If not met → suspicious; consider PET or biopsy.
Rule out pheochromocytoma biochemically before biopsy or surgery in any mass that is heterogeneous, >3 cm, hemorrhagic, or cystic. Plasma metanephrines or 24-hour urine catecholamines should be checked first.
Size and Growth Cutoffs
| Size / Clinical Context | Recommendation |
|---|---|
| <4 cm, indeterminate, no known malignancy | Washout CT or chemical shift MRI; if benign → no further imaging |
| ≥4 cm or growing (>1 cm/yr or >20% increase) | Surgical referral regardless of imaging characteristics |
| Any size, known extra-adrenal malignancy | PET or biopsy (after ruling out pheo biochemically) |
| Suspected pheochromocytoma | Biochemical evaluation before any intervention; MIBG or DOPA-PET if needed |
| Myelolipoma (<4 cm, classic macroscopic fat −30 to −100 HU) | No follow-up |
| Simple adrenal cyst (<4 cm, no internal structure) | No follow-up |
Special Lesion Types
Lipid-poor adenoma: Attenuation 11–30 HU on unenhanced CT. Cannot be diagnosed on HU alone — requires washout CT or chemical shift MRI showing signal dropout on out-of-phase images.
Myelolipoma: Macroscopic fat (−30 to −100 HU), often heterogeneous due to myeloid elements. No follow-up if <4 cm and typical. Surgical referral if symptomatic (hemorrhage) or large.
Adrenal cyst: Simple, water-density, no enhancement. No follow-up if <4 cm.
Pheochromocytoma: Can be any HU, often heterogeneous with cystic/hemorrhagic areas. Do not biopsy before biochemical exclusion — hypertensive crisis risk.
Why This Matters
Adrenal incidentalomas are found in 1–4% of abdominal CTs. The vast majority are benign, nonfunctioning adenomas. The HU cutoff of ≤10 identifies lipid-rich adenomas with ~98% specificity, allowing most patients to be discharged with no further imaging in a single read. The key risk factors that warrant expedited evaluation are large size (≥4 cm), interval growth, and known extradrenal malignancy.
Reference
Mayo-Smith WW, Song JH, Boland GL, et al. Management of Incidental Adrenal Masses: A White Paper of the ACR Incidental Findings Committee. J Am Coll Radiol. 2017;14(8):1038–1044.