Adrenal washout
Calculate absolute and relative adrenal washout from unenhanced, portal venous and delayed CT attenuation, with the thresholds that separate an adenoma from an indeterminate lesion.
Attenuation per phase
Absolute washout
74%
Relative washout
51%
Absolute washout 74% · relative washout 51%
Absolute washout is 74%, above the 60% threshold. In a homogeneous, well-defined lesion that fits an adenoma.
Relative washout of 51% is above the 40% threshold and points the same way.
The criteria were validated on homogeneous lesions without necrosis or haemorrhage. In an irregular, heterogeneous or growing lesion the percentages say little, however neatly they come out.
And the pitfall you cannot afford to miss: a phaeochromocytoma can show washout above 60% and pass as an adenoma. Hypervascular metastases, from renal cell carcinoma for instance, sometimes wash out too. Always weigh the biochemistry and clinical context.
How to read the result
Washout asks a single question: does this lesion lose contrast the way adrenal cortical tissue does? A result above the thresholds supports an adenoma. A result below them is indeterminate, not negative — an important distinction when writing the conclusion.
The formulas
APW = (portal − delayed) ÷ (portal − unenhanced) × 100
RPW = (portal − delayed) ÷ portal × 100
Enter attenuation in Hounsfield units. Portal venous is acquired around 60–70 seconds and the delayed phase at 15 minutes; a shorter delay lowers the calculated washout and makes an adenoma look indeterminate.
An unenhanced attenuation of 10 HU or less is diagnostic for a lipid-rich adenoma on its own, and washout is then unnecessary.
When to use it
- Characterising an incidentally found adrenal nodule on a dedicated adrenal protocol CT.
- Deciding whether a lesion in a patient with a known primary tumour can be called benign.
- Reporting a lesion where the unenhanced attenuation sits above 10 HU and washout is the deciding measurement.
- Working with an outside study that lacks an unenhanced series, where only relative washout is available.
⚠️ Pitfalls
- A phaeochromocytoma can wash out above 60% and pass as an adenoma. Washout never overrules biochemistry or a suggestive clinical picture.
- Hypervascular metastases, notably from renal cell carcinoma, may also wash out. In a patient with such a primary, treat a washout result with more caution than usual.
- The criteria were validated on small, homogeneous lesions. Necrosis, haemorrhage or a heterogeneous lesion invalidates the calculation even when the arithmetic works.
- Place the ROI over the bulk of the lesion, in the same position on all three series, and avoid the rim. Inconsistent ROI placement is the commonest reason two readers disagree on washout.
- A delayed phase acquired earlier than 15 minutes systematically underestimates washout and pushes adenomas into the indeterminate band.
Frequently asked questions
What is the formula for absolute adrenal washout?
Subtract the delayed attenuation from the portal venous attenuation, divide by the portal venous minus the unenhanced attenuation, and multiply by 100. Values of 60% or more are consistent with an adenoma.
When should I use relative washout instead?
When there is no unenhanced series, which is common with outside studies. Relative washout uses only the portal venous and delayed values, with a threshold of 40%. Where both are available, absolute washout is the better validated measure.
Does a low washout mean the lesion is malignant?
No. It means the lesion is not characterised. Many benign lesions fail the criteria, so the correct conclusion is indeterminate, followed by chemical-shift MRI, comparison with prior imaging or further work-up.
Can a phaeochromocytoma show adenoma-like washout?
Yes, and this is the classic trap. A washout above 60% does not exclude a phaeochromocytoma, so plasma or urinary metanephrines remain necessary when the clinical picture raises the question.
Why does the delayed phase have to be at 15 minutes?
The thresholds were derived from 15-minute delayed imaging. An earlier delay leaves more contrast in the lesion, lowers the calculated washout and turns genuine adenomas into indeterminate results.
References
📄Korobkin M, et al. Delayed enhanced CT for differentiation of benign from malignant adrenal masses. Radiology. 1996;200(3):737–742.
📄Caoili EM, et al. Adrenal masses: characterization with combined unenhanced and delayed enhanced CT. Radiology. 2002;222(3):629–633.
📄Boland GW, et al. Adrenal masses: characterization with delayed contrast-enhanced CT. Radiology. 1997;202(3):693–696.
📄ACR Incidental Findings Committee white paper on adrenal masses.↗This calculator supports clinical judgement and does not replace it. Always check the result against your own measurements and the guideline in force.
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