Incidental Discovery of an Adrenal Mass
Found by chance on a scan ordered for something else entirely.
- ~5%: Incidence on abdominal CT (rises with age)
- ~10-15%: Bilateral masses (of incidentalomas)
- >80%: Most are benign (nonfunctioning adenomas)
- 50-70: Peak detection age (years old)
What counts as an incidentaloma
A mass ≥1cm found on imaging not aimed at the adrenal gland.
Why prevalence is rising
Wider CT/MRI use for unrelated complaints detects more incidental masses.
Every incidentaloma needs a structured workup, even if it looks harmless.
Initial framing questions
Is it benign or malignant? Is it hormonally active? Does size matter?
CT Density (Hounsfield Unit) Assessment
Unenhanced CT attenuation separates lipid-rich benign lesions from denser ones.
- <10 HU: Benign cutoff (lipid-rich adenoma)
- 10-20 HU: Indeterminate zone (needs further imaging)
- >20 HU: Concerning threshold (malignancy possible)
- ~71%: Adenoma sensitivity (at <10 HU cutoff)
Reading the Hounsfield scale
Fat reads dark, dense tissue reads bright on unenhanced CT.
Low-HU adenomas
Lipid-rich content gives low attenuation and reassures benignity.
Below 10 HU is a strong, validated marker of a benign adenoma.
When density is ambiguous
Indeterminate or high HU prompts washout CT, MRI, or biopsy.
Hormonal Function Screening
Every incidentaloma is tested for hormone excess, no matter how it looks.
- ~5-30%: Autonomous cortisol (of incidentalomas)
- ~5%: Pheochromocytoma (catecholamine excess)
- ~1-2%: Aldosteronoma (Conn) (if hypertensive)
- 1mg DST: Screening test (dexamethasone suppression)
Cortisol excess screening
Overnight dexamethasone suppression test flags autonomous secretion.
Catecholamine excess
Plasma or urine metanephrines rule out pheochromocytoma before biopsy.
Never biopsy an adrenal mass before excluding pheochromocytoma.
Aldosterone excess
Aldosterone-renin ratio is checked in hypertensive or hypokalemic patients.
Mass Size and Malignancy Risk
Bigger masses carry a meaningfully higher chance of malignancy.
- <4 cm: Low-risk size (lower malignancy odds)
- 4-6 cm: Intermediate (individualized decision)
- >6 cm: High-risk size (surgery favored)
- ~25%: ACC risk at >6cm (adrenocortical carcinoma)
Size as a risk multiplier
Larger diameter correlates with rising carcinoma probability.
Surgical threshold
Masses over 4cm are often favored for resection, especially with other risk features.
Size alone rarely decides — it is weighed with density and function.
Growth on follow-up
Interval growth on repeat imaging also raises surgical priority.
Combined Risk & Management Decision
Imaging, hormone status, and size together choose the care pathway.
- 6-12 mo: Surveillance imaging (low-risk masses)
- MRI/PET: Further workup (indeterminate cases)
- Adrenalectomy: Surgical referral (high-risk features)
- Annual: Repeat hormone testing (up to 5 years)
Low-risk pathway
Small, low-HU, nonfunctioning masses go to surveillance imaging.
Intermediate pathway
Indeterminate density or borderline size prompts further workup.
A single high-risk feature can override an otherwise reassuring picture.
High-risk pathway
Functioning tumors, large size, or high HU favor surgical removal.