Adrenal adenomas are the most common lesion of the adrenal gland, usually discovered incidentally as well-circumscribed hypodense lesions with variable lipid content.
Description
Adrenal adenomas are benign tumours arising from the cortex of the adrenal gland. They can be functioning, producing excess hormones such as cortisol, aldosterone, or androgens, or non-functioning, with no excess hormone production. These adenomas are the most common adrenal mass in adults, particularly those over 40 years of age, and occur slightly more often in females.
Pathogenesis
Adrenal adenomas result from clonal proliferation of adrenal cortical cells, though the exact trigger remains unclear. Functioning adenomas are often associated with mutations in various genes involved in steroid biosynthesis or cellular proliferation. Non-functioning adenomas may also result from clonal proliferation but without associated genetic mutations leading to hormone overproduction.
Subtypes
Adrenal adenomas can be categorised into:
- Non-functioning adenoma: Most common subtype. These do not produce excess hormones and are often detected incidentally (‘incidentalomas’).
- Cortisol-producing adenoma: May lead to Cushing syndrome.
- Aldosterone-producing adenoma: May lead to Conn syndrome.
- Androgen-producing adenoma: Rare, may cause virilisation or feminisation depending on the specific hormones produced.
Epidemiology, Risk Factors & Associations
- The prevalence of adrenal adenomas increases with age and they are more common in women (2:1 ratio).
- Association with several genetic syndromes, such as Multiple Endocrine Neoplasia type 1 (MEN1), Beckwith-Wiedemann syndrome, and Carney complex.
Clinical Features
The majority of adrenal adenomas are asymptomatic and discovered incidentally. Symptoms, when present, are usually due to excess hormone production
- Cushing syndrome (e.g., moon face, buffalo hump, abdominal striae, centripetal obesity, easy bruising)
- Conn syndrome (e.g., hypertension, hypokalemia, muscle weakness, frequent urination)
- Androgen excess (e.g., acne, deepened voice, hirsutism in women, early puberty in children or irregular menses in women).
Complications
Risk of malignant transformation is low. Functioning adenomas can lead to significant morbidity due to the effects of excess hormone production.
Pathological Features
Histopathology
- Macroscopic: Adenomas are generally small (less than 4 cm), well-circumscribed, and yellow due to lipid content.
- Microscopic: Well-differentiated cells arranged in cords or clusters, resembling normal adrenal cortical tissue.
Serology
- Hormonal assays depending on the type of functioning adenoma (cortisol, aldosterone, androgens).
Biochemistry
- May show abnormalities in potassium, glucose, and lipid levels due to excess hormone production.
Radiological Features
General Features
- Characteristically demonstrates well-circumscribed, round or oval, and homogenous lesions.
- Calcification is uncommon (less than 10% of cases).
- The ‘adrenal adenoma attenuation’ sign, referring to low attenuation on non-contrast CT due to high lipid content.
CT
- Non-contrast: Typically, an adrenal adenoma has a low attenuation value (less than 10 Hounsfield units) due to high intracellular lipid content.
- C+ Arterial and C+ Venous: Hypodense compared to the liver, washout characteristics can further differentiate adenomas from metastases.
MRI
- T1: Adenomas have variable signal intensity due to variable lipid content. High lipid content adenomas appear bright.
- T2: Typically hypointense.
- In-Out-Phase: High lipid content adenomas lose signal on out-of-phase imaging due to intracellular microscopic fat.
US
- B-mode: Hypoechoic, well-defined lesion.
- Colour: Generally avascular or hypovascular.
NM
- PET FDG: Typically low uptake.
Diagnosis
The diagnosis is made based on a combination of imaging characteristics and biochemical tests, with the demonstration of a small, low-attenuation lesion on non-contrast CT being highly suggestive.
Differential Diagnosis
- Metastases: more commonly irregular and heterogeneous, often with higher attenuation on non-contrast CT, and slower washout on contrast-enhanced CT.
- Adrenal carcinoma: typically larger with irregular borders, heterogeneous enhancement, and areas of necrosis or haemorrhage.
- Pheochromocytoma: Mass with cystic portion that is T2 light bulb bright often larger but less than 6 cm. Calcification is unlikely.
Management
Management depends on the type and symptoms. Non-functioning adenomas without concerning features may be managed conservatively with monitoring. Functioning adenomas or those with concerning features typically require surgical resection.