CT Adrenal Protocol
On this page
Direct answer
An adrenal nodule discovered by chance is diagnosed by fat and by washout. On unenhanced CT, a homogeneous attenuation of 10 Hounsfield units or less identifies a lipid-rich adenoma and ends the workup. If the lesion is denser, a contrast study with washout calculation is performed: absolute washout of 60 per cent or more, or relative washout of 40 per cent or more on a delayed phase at about 60 seconds after a 60-70 second portal phase, is adenoma-like; lower washout is indeterminate and raises metastasis or carcinoma. Biochemistry — metanephrines before any biopsy — and size thresholds, 4 cm watched closely and 6 cm generally resected, complete the algorithm every NEET-PG vignette is built on.
What you must remember
- Unenhanced attenuation ≤ 10 HU: lipid-rich adenoma, specific enough to stop; most adenomas are cleared here, but the cut-off misses lipid-poor adenomas.
- Washout arithmetic: absolute percentage washout = (enhanced − delayed)/(enhanced − unenhanced) × 100, with ≥ 60 per cent adenoma; relative percentage washout = (enhanced − delayed)/enhanced × 100, with ≥ 40 per cent adenoma. An unenhanced phase is needed only for the absolute calculation.
- Never biopsy an adrenal lesion before excluding pheochromocytoma with plasma or urinary metanephrines — biopsy of an unsuspected phaeochromocytoma can precipitate a hypertensive crisis.
- Size rules: incidentalomas under 4 cm with benign features are followed, 4-6 cm individualised, and above 6 cm generally excised because of carcinoma risk; growth of more than 20 per cent plus 5 mm on follow-up is concerning.
- Macroscopic fat (large negative HU values): myelolipoma — diagnosis made, no follow-up needed.
- Bilateral adrenal masses: think metastases (lung, breast, melanoma), bilateral phaeochromocytoma (MEN 2, VHL), hyperplasia, and in India adrenal tuberculosis — necrotic low-attenuation rims and calcification in a patient with Addison disease is a classic presentation.
- Indeterminate lesions: homogeneous, small, biochemically silent lesions may be followed with imaging at 6-12 months; heterogeneity, irregular margins and necrosis push toward 18F-FDG PET or resection.
Working the algorithm on one patient
A 55-year-old man has a left adrenal nodule found on CT done for renal colic, 3 cm, smooth. Step one is confirm it is truly incidental and assess hypertension, virilisation and Cushing signs clinically. Step two is biochemistry: dexamethasone suppression testing for cortisol, metanephrines for phaeochromocytoma, and in a young hypertensive, aldosterone-to-renin ratio. Step three is the CT protocol. The unenhanced scan shows attenuation of 6 HU — lipid-rich adenoma, stop, reassure. Had it been 25 HU, the study continues: post-contrast at 60-70 seconds (say 90 HU) and delayed at 60 seconds (say 40 HU). Absolute washout = (90 − 40)/(90 − 25) × 100 = 77 per cent, comfortably adenoma. Had the same lesion washed out only 25 per cent, it is indeterminate — and the next questions are the primary tumour history and the FDG-PET, with adrenal vein sampling reserved for functioning cortical tumours.
The Indian twist deserves its own sentence: a 30-year-old with weight loss, darkening skin and low cortisol who shows bilateral bulky adrenals with peripheral rim enhancement and scattered calcification has tuberculosis far more often than in Western series, and the answer to the imaging question is adrenal TB until proven otherwise, with biopsy sometimes justified.
Where the exam sets its traps
The percentages are the trap. Candidates quote "more than 50 per cent washout" generically, but the exam expects the split — 60 absolute, 40 relative — and knows the absolute formula subtracts the unenhanced value from both terms. A second trap is the washout mimic: phaeochromocytomas also wash out like adenomas, which is exactly why metanephrines come first; the exam vignette of a hypertensive patient with an "adrenal adenoma on washout" is a phaeochromocytoma question. Third, density heterogeneity invalidates the HU rule — the measurement is valid only on homogeneous lesions, so measure the whole lesion, not a pixel. Finally, do not call every incidentaloma cancer: the overwhelming majority are non-functioning adenomas, and the exam rewards the calm, size-stratified answer over the reflex to resect.
Frequently asked questions
What attenuation value on unenhanced CT diagnoses a lipid-rich adrenal adenoma?
Ten Hounsfield units or less in a homogeneous lesion is diagnostic, because intracytoplasmic lipid lowers attenuation; no further characterisation is required.
How are absolute and relative percentage washout calculated?
Absolute washout is (enhanced − delayed)/(enhanced − unenhanced) × 100 with ≥ 60 per cent indicating adenoma, and relative washout is (enhanced − delayed)/enhanced × 100 with ≥ 40 per cent indicating adenoma.
Why must pheochromocytoma be excluded before adrenal biopsy?
Untreated phaeochromocytoma can release catecholamines during needle manipulation and precipitate a hypertensive crisis, so plasma or urinary metanephrines are mandatory first.
What size thresholds guide management of adrenal incidentalomas?
Lesions under 4 cm with benign imaging are followed, 4-6 cm are individualised, and lesions 6 cm or larger are generally resected because adrenal cortical carcinoma risk rises with size.
Which adrenal imaging appearance is typical of tuberculosis in India?
Bilaterally enlarged glands with low-attenuation necrotic centres, peripheral rim enhancement and, later, calcification, seen in a patient with adrenal insufficiency, is characteristic of adrenal tuberculosis.