Incidental adrenal lesion (adrenal incidentaloma) is not an uncommon finding on abdominal imaging (e.g. CT or MRI) with an incidence of approximately 5% in western countries. The majority of adrenal incidetalomas are benign – approximately 80% are non-functioning adenomas, 5% cortisol producing adenoma, 5% phaeochromocytoma, 1% aldosteronoma, and the rest consisting of adrenocortical cancers/metastases, cysts, ganglioneuromas and myelolipomas.

The initial investigation and management of adrenal incidentaloma can be simplified into the following:

1. Does the adrenal look malignant? CT (and sometimes MRI) scan is extremely useful in determining whether an adrenal incidentaloma looks benign, indeterminate, or frankly malignant.
   • A homogenous adrenal lesion with regular margins and Hounsfield unit <10 on CT scan is suggestive of a benign lipid rich adenoma, accounting for 70% of benign adrenal adenoma.
   • A heterogenous adrenal lesion with irregular margins is suspicious for malignancy. Other features of malignancy include invasion of local structures (e.g. kidney, liver, inferior vena cava). If someone has a history of cancer, adrenal metastases must also be considered. Adrenal gland is the fourth most common sites of distant metastases (after lung, liver and bone).
   • Lesions that do not fit the above 2 groups needs to be investigated with contrast-enhanced CT scan to assess “contrast washout” in the delayed phase. Benign adrenal adenomas enhance rapidly, but the contrast washes out briskly as well. Adenomas that has a 60% absolute contrast washout or 40% relative contrast washout are generally considered benign. A useful adrenal washout calculator can be found here.
   • PET scan is useful to exclude adrenal malignancy/metastases with more than 70% sensitivity and specificity, and DOTATE-PET is particularly useful for pheochromocytoma. PET scan however is generally not needed for most cases of adrenal incidentaloma.
2. Is the adrenal lesion metabolically active i.e. producing hormones (cortisol, aldosterone, sex hormones and catecholamines e.g. adrenaline and noradrenaline)?
   • Hypercortisolism – 1 mg overnight dexamethasone suppression test
   • Phaeochromocystoma – 24-hour urinary metanephrines and normetanephrines
   • Aldosteronoma – plasma aldosterone:renin ratio
   • Sex hormones – very rare. Test only if patient display virilisation features. If present, be suspicious for adrenal malignancy.

(Confounders – drugs e.g. steroids, ACE inhibitors, SSRIs can interfere with test results and need to be withheld temporarily)

3. How big is the lesion and is it growing?
   • Size is an important predictor for malignancy. Numerous studies have shown that the risk for adrenal malignancy is approximately 5-10% for lesions 4-6 cm, and 15-20% for lesions >6 cm. Therefore, any lesions larger than 4cm should be referred to an endocrine surgeon for consideration of surgery.
   • If an adrenal incidentaloma is <4cm, looks benign on imaging, and is not metabolically active (i.e. not secreting any hormones), it can be followed-up with imaging (see below).

Surgery

Most adrenal tumours nowadays can be removed endoscopically – the two commonest method being laparoscopic transabdominal adrenalectomy, and retroperitoneoscopic adrenalectomy (RPA).

Laparoscopic transabdominal adrenalectomy is similar to other laparoscopic surgery e.g. cholecystectomy or appendicectomy. The advantage of this technique is that most surgeons are familiar with the anatomy. The disadvantages (again like other laparoscopic procedures) are that transabdominal approach potentially causes ileus, and adhesions which may predispose to adhesive small bowel obstruction in the future.

RPA is a technique developed in Germany, and now is popular across the world for adrenalectomy. The difference here is that patients are placed prone in a kneeling position and the approach to the adrenal gland is posteriorly from the back. The main advantage of this technique is that patients recover exceptionally well and quickly post-operatively, and most patient can be discharged within 24 hours with minimal pain. As there is no breach of the peritoneal cavity, issues such as post-operative ileus and adhesion formation are avoided.

Open adrenalectomy is usually reserved for adrenocortical carcinoma, metastases, or large lesion to minimize the risk of breaching the tumour and causing spillage and seeding of the cancer cells. Whilst size is not an independent predictor for open surgery (factors e.g. location of tumour and body habitus of the patient play a significant role), it is usually very challenging to resect an adrenal tumour endoscopically when larger than 8 cm.  

Follow-up

For benign adrenal tumours that are not resected, it is reasonable to obtain a non-contrast CT in 6-12 months to assess for growth in size. Tumours that grow >20% or >5mm per year should be referred to an endocrine surgeon for consideration of resection. Lipid rich (Hounsfield unit <10) tumours <4 cm and remain stable in size do not require any further investigation or intervention.

Further reading:

1. 1. Management of adrenal incidentalomas: European Society of Endocrinology Clinical Practice Guideline in collaboration with the European Network for the Study of Adrenal Tumors. Eur J Endocrinol. 2016 Aug;175(2): G1-G34. doi: 10.1530/EJE-16-0467.
   2. Incidental adrenal masses – A primary care approach. Australian Family Physician. Volume 46, No.6, June 2017 Pages 385-390