🩺 Adrenal Washout Calculator

Most adrenal lumps found incidentally are benign adenomas, but telling them from cancers and pheochromocytomas matters. CT washout exploits a behavioral difference: adenomas take up contrast and then clear it quickly, while non-adenomas hold onto it. Measure attenuation in Hounsfield units at three phases, unenhanced, portal venous, and 15-minute delayed, and the washout percentages sort most lesions correctly.

The thresholds: absolute washout of 60% or more, or relative washout of 40% or more, indicates adenoma. This calculator computes both from the three attenuation values.

How to use this calculator

Enter the lesion's attenuation in Hounsfield units at each CT phase.

The formulas

Absolute washout uses all three phases; relative washout skips the unenhanced phase for scans where it was not acquired.

Either criterion meeting threshold supports adenoma; the pair agrees in most cases. Sensitivity and specificity both run near 90% in validation studies, which is why washout CT is the workhorse adrenal workup.

Worked example

Unenhanced 20 HU, portal venous 80 HU, delayed 40 HU. Absolute washout = (80 - 40) / (80 - 20) x 100 = 40/60 x 100 = 66.7%. Relative washout = 40/80 x 100 = 50%. Both clear their thresholds: adenoma pattern.

Contrast a non-adenoma: unenhanced 35, portal 90, delayed 70. Absolute = 20/55 = 36.4%; relative = 20/90 = 22.2%. Both below threshold: non-adenoma pattern needing further workup.

What the patterns mean

Adenoma pattern: in the right clinical setting (small, homogeneous lesion, no cancer history), this usually ends the workup with imaging surveillance or nothing further. Non-adenoma pattern: triggers the differential: metastasis (especially with known primary cancer), adrenocortical carcinoma (usually large), pheochromocytoma, or hemorrhage.

Washout is one test, not the diagnosis. Lesion size, contour, heterogeneity, unenhanced attenuation under 10 HU (lipid-rich adenoma, diagnosable without washout), hormone workup, and clinical history all feed the final call. For related imaging math, see our bladder volume calculator.

Limits and mimics

Washout needs all three phases done right: mistimed delayed images or ROI placement over necrosis corrupts the numbers. Pheochromocytomas can occasionally show adenoma-like washout, which is why catecholamine screening precedes biopsy or surgery for any suspicious lesion. Hemorrhage and cystic change also distort attenuation.

Small lesions under 1 cm suffer partial-volume averaging, making HU values unreliable. And washout characterizes the lesion, not its hormone activity: a 2 cm adenoma still needs aldosterone, cortisol, and catecholamine screening per guidelines.

Why adenomas wash out fast

The washout difference is plumbing. Adenomas are benign cortical tumors with a rich but orderly capillary network: contrast floods in during the arterial and portal phases, then drains promptly through normal venous channels. Malignant lesions and pheochromocytomas have chaotic, leaky vasculature with areas of poor drainage, so contrast pools and lingers into the delayed phase.

Lipid content adds a second signature. Many adenomas are lipid-rich, measuring 10 HU or less unenhanced, which diagnoses them without any washout math. Lipid-poor adenomas look denser and need the washout test, but they share the same orderly vasculature, so they wash out the same way. The biology is consistent even when the attenuation is not.

This is also why the 15-minute delay matters: too early and even adenomas have not cleared; too late and the distinction blurs as everything equilibrates. Protocols standardize the timing so the thresholds mean the same thing everywhere.

One nuance: hypervascular metastases, especially from renal cell carcinoma, can wash out briskly and mimic adenomas, which is why a known primary cancer changes the interpretation of the same numbers. Size helps: lesions over 4 cm are statistically more likely malignant regardless of washout, and guidelines lean toward resection or close surveillance at that size.

For patients, the bottom line is reassuring: most adrenal incidentalomas are benign adenomas, and the washout test resolves the question without a needle in most cases.

What happens after an indeterminate lesion

Not every lesion sorts neatly. When washout lands in the gray zone, or the lesion has suspicious features (large size, irregular borders, heterogeneity), the workup continues. Options include short-interval follow-up CT or MRI to watch for growth, MRI chemical-shift imaging (which detects microscopic fat like unenhanced CT does), and FDG-PET for patients with known cancer.

Hormone workup runs in parallel and matters regardless of imaging: every adrenal incidentaloma gets screened for excess cortisol, aldosterone (if hypertensive), and catecholamines (if pheochromocytoma is possible). A nonfunctioning adenoma and a cortisol-secreting one look identical on washout but are managed completely differently.

Surgery enters for large lesions (generally over 4 cm), growing lesions, hormone-secreting tumors, and anything suspicious for cancer. Most small nonfunctioning adenomas need nothing but periodic imaging or nothing at all. The goal of the whole cascade is to find the few that matter without operating on the many that do not.

For informational purposes only, not medical advice. Consult a healthcare professional.