Adrenal Washout Calculator (absolute and relative CT washout)

Understanding the adrenal washout calculator — concept and principle

This calculator works out the absolute and relative percentage washout of an adrenal nodule from adrenal-protocol CT attenuation values, and shows how the result relates to the thresholds in the 2017 American College of Radiology white paper. Radiology trainees, radiologists and clinicians reviewing reports use it.

How fast contrast leaves the mass — with the default inputs: 75% washout.

How it is calculated

Absolute percentage washout APW = (enhanced − delayed) ÷ (enhanced − unenhanced) × 100
Relative percentage washout RPW = (enhanced − delayed) ÷ enhanced × 100 (used when there is no unenhanced scan)
Attenuation values are in Hounsfield units (HU): unenhanced, portal venous (about 60–75 s after contrast) and delayed (commonly 15 min)
ACR 2017: unenhanced 10 HU or less = lipid-rich adenoma; APW 60% or more or RPW 40% or more = consistent with adenoma

The principle

Adrenal adenomas take up contrast and wash it out quickly, whereas many other lesions retain contrast longer. Comparing attenuation before, during and after contrast measures that behaviour. Lipid-rich adenomas are already identified by low attenuation (10 HU or less) on the unenhanced scan because fat lowers attenuation; washout helps classify lipid-poor adenomas. Some pheochromocytomas, metastases and carcinomas can wash out like adenomas, so hormone tests and the full report matter.

Worked example

Defaults: unenhanced 20 HU, enhanced 80 HU, delayed 35 HU.

APW = (80 − 35) ÷ (80 − 20) × 100 = 45 ÷ 60 × 100 = 75%.
RPW = (80 − 35) ÷ 80 × 100 = 56%.
Unenhanced 20 HU is above 10, but APW 75% ≥ 60%: Washout consistent with adenoma.

Each point is the calculator re-run with only “Delayed attenuation (HU)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Delayed attenuation (HU)	Absolute washout
8.75	119%
17.5	104%
26.2	90%
35	75%
43.8	60%
52.5	46%
70	17%

How to read the result

Where does your result fall?

Washout above the thresholds supports a benign adenoma in the right clinical setting. Lower washout is indeterminate and leads to follow-up imaging, PET-CT, biopsy or surgery depending on the situation. The result is one piece of the radiologist's assessment, alongside size, appearance, growth and hormone tests.

Common mistakes

Using a delayed phase other than the protocol time.
Measuring regions of interest that include necrosis or calcification.
Using RPW when an unenhanced scan is available.
Ignoring hormone testing for functioning tumours.

Keep in mind: Not a diagnosis. Imaging results must be interpreted by a radiologist and the treating team.

Frequently asked questions

What absolute washout suggests an adenoma?

60% or more, according to the ACR 2017 white paper.

When is relative washout used?

When no unenhanced scan is available; 40% or more is consistent with adenoma.

What does 10 HU or less mean?

On an unenhanced scan it indicates a lipid-rich adenoma.

Sources: Mayo-Smith WW, Song JH, Boland GL, et al., "Management of incidental adrenal masses: a white paper of the ACR Incidental Findings Committee", J Am Coll Radiol 2017;14:1038–1044

Checked 9 Nov 2026. Mayo-Smith WW, Song JH, Boland GL, et al., “Management of incidental adrenal masses: a white paper of the ACR Incidental Findings Committee”, J Am Coll Radiol 2017;14:1038–1044 — thresholds (≤ 10 HU unenhanced; APW ≥ 60%, RPW ≥ 40%) as summarised by Radiopaedia; washout formulas standard.

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