Adrenal adenoma

Key points

• Most common adrenal lesion; ~80% are lipid-rich (intracytoplasmic fat)
• Unenhanced CT attenuation ≤10 HU is diagnostic of lipid-rich adenoma (Sp ~98%)
• Chemical shift MRI (signal drop on opposed-phase) confirms intracellular lipid
• Contrast washout CT distinguishes lipid-poor adenomas from metastases (APW >60%, RPW >40%)
• Most are non-functioning incidentalomas; always exclude phaeochromocytoma and hypercortisolism

Epidemiology & risk factors

• Prevalence ~3-7% on abdominal CT (autopsy series up to 9%)
• Incidence increases with age; slightly more common in females
• Higher prevalence in obesity, DM, hypertension
• Associated with conditions causing adrenal hyperplasia (e.g., congenital adrenal hyperplasia)

Pathophysiology

• Benign cortical neoplasm composed of lipid-laden adrenocortical cells
• ~80% lipid-rich (abundant intracytoplasmic cholesterol/fat); ~20% lipid-poor (fewer lipid vacuoles, diagnostic challenge)
• ~15% are functioning: Conn syndrome (aldosterone), Cushing syndrome (cortisol), virilisation (androgens)

Clinical features

• Vast majority are incidental findings (adrenal incidentaloma)
• Functioning adenomas present with relevant endocrine syndrome
• Size typically <4 cm; larger lesions raise concern for adrenocortical carcinoma

Imaging / investigations

CT (unenhanced)

• ≤10 HU - diagnostic of lipid-rich adenoma (no further workup needed)
• >10 HU - indeterminate; proceed to washout CT or chemical shift MRI

CT (contrast washout)

• Used for lipid-poor adenomas (>10 HU on unenhanced)
• Measure attenuation at 60-70 sec (portal venous) and 15 min (delayed)
• APW (absolute percentage washout) >60% or RPW (relative percentage washout) >40% - adenoma
• Adenomas wash out rapidly vs metastases which retain contrast

MRI

• Chemical shift imaging (in-phase/opposed-phase GRE) - key sequence
  • Signal drop on opposed-phase = intracellular lipid = adenoma
  • Quantify with adrenal-to-spleen signal intensity index (ASI) >16.5% - adenoma
• T1: iso- to low signal
• T2: low to intermediate signal (metastases typically bright on T2)
• Lipid-poor adenomas may not show signal drop - use CT washout instead

Nuclear medicine

• FDG PET: adenomas are typically FDG-avid similar to or less than liver
• Metastases show increased FDG uptake relative to liver
• NP-59 (iodocholesterol) scintigraphy: concordant uptake in functioning adenoma

Biochemical workup (for incidentalomas)

• 1 mg overnight dexamethasone suppression test (Cushing)
• Plasma aldosterone/renin ratio (if hypertensive - Conn)
• Plasma/urinary metanephrines (exclude phaeochromocytoma)

Differentials

• Metastasis - irregular enhancement, high T2, no signal drop on chemical shift, no washout, often bilateral, known primary
• Phaeochromocytoma - light bulb bright on T2, avid enhancement, elevated metanephrines
• Adrenocortical carcinoma - large (>4 cm), heterogeneous, irregular margins, invasion, calcification
• Myelolipoma - contains macroscopic fat (negative HU areas on CT), no diagnostic confusion with chemical shift
• Lymphoma - bilateral, homogeneous, low T2, clinical context

Management

• ≤10 HU and <4 cm - benign, no follow-up needed (ACR)
• Indeterminate (>10 HU, <4 cm) - CT washout or chemical shift MRI
• ≥4 cm or growing >1 cm/year - consider surgical resection (risk of carcinoma)
• Functioning adenoma - surgical resection (laparoscopic adrenalectomy)
• Follow-up of indeterminate lesions: repeat imaging at 6-12 months