Free Adrenal Washout Calculator

What Is an Adrenal Washout Calculator?

The adrenal washout calculator (often referred to as an adrenal CT calculator or washout calculator CT) assesses how quickly an adrenal lesion clears intravenously administered contrast. It relies on Hounsfield unit (HU) measurements taken at three time points: a non‑contrast (pre‑contrast) scan, a contrast‑enhanced scan acquired 60–75 seconds after injection, and a delayed scan obtained 15 minutes later. By applying both the absolute and relative washout formulas, the tool helps distinguish adrenal adenomas—the most common benign adrenal tumor—from other lesions such as metastases, pheochromocytomas, or malignant neoplasms.

Because many adenomas are discovered incidentally during abdominal imaging for unrelated indications, this adrenal adenoma calculator is also frequently called an adrenal incidentaloma calculator. It serves as a quick, non‑invasive aid in the workup of these incidental findings.

The Protocol: Three Essential CT Phases

The washout protocol requires three distinct acquisitions:

• Unenhanced (pre‑contrast) phase – performed before any contrast agent is administered.
• Contrast‑enhanced phase – acquired 60–75 seconds after contrast injection (portal venous phase).
• Delayed phase – obtained after a 15‑minute delay.

All density values are recorded in Hounsfield units (HU), the standard scale for tissue density in computed tomography.

Interpreting the Results

After the washout percentages are calculated, the following radiological thresholds are commonly applied:

Measure	Threshold	Clinical Significance
Absolute washout	> 60%	Highly suggestive of adrenal adenoma
Relative washout	> 40%	Highly suggestive of adrenal adenoma
Pre‑contrast HU	> 120 HU (any lesion)	Not compatible with adenoma; consider alternative pathology
Pre‑contrast HU (non‑calcified, non‑hemorrhagic)	> 43 HU	Suspicious for malignancy irrespective of washout

Additional caveats worth remembering:

• Pheochromocytomas can occasionally exhibit contrast washout, but intracytoplasmic lipid sufficient to lower non‑contrast HU below 10 HU is extremely rare in these tumors.
• In patients with renal cell carcinoma, hepatocellular carcinoma, or other hypervascular metastases, the specificity of washout measurements may be reduced.
• Washout characteristics should not be applied to lesions that are inhomogeneous or contain large necrotic or hemorrhagic areas.

The Washout Formulas

The two washout indices are computed from the same three HU values (pre = pre‑contrast, post = post‑contrast, delayed = delayed phase).

Absolute washout (often provided by an absolute washout calculator):

$$\text{Absolute} = 100 \times \frac{\text{post} - \text{delayed}}{\text{post} - \text{pre}}$$

Relative washout (often provided by a relative washout calculator):

$$\text{Relative} = 100 \times \frac{\text{post} - \text{delayed}}{\text{post}}$$

Both are expressed as percentages. The tool performs these calculations instantly, eliminating manual arithmetic.

Practical Example

Consider a patient with the following HU values:

Phase	HU
Pre‑contrast	32
Post‑contrast (60 s)	192
Delayed (15 min)	105

Absolute washout:

$$\frac{192 - 105}{192 - 32} \times 100\% = \frac{87}{160} \times 100\% = 54.4\%$$

Relative washout:

$$\frac{192 - 105}{192} \times 100\% = \frac{87}{192} \times 100\% = 45.3\%$$

The relative washout (45.3%) exceeds the 40 % threshold, so this lesion would be considered highly suggestive of an adrenal adenoma.

Background: Adrenal Adenomas

An adrenal (adrenocortical) adenoma is a benign growth of the adrenal cortex. It can be non‑functioning (hormonally inactive) or functioning (secreting excess cortisol, aldosterone, or androgens). Most adrenal adenomas are asymptomatic and discovered incidentally during imaging, hence the term “adrenal incidentaloma.”

Epidemiological data from CT and autopsy studies report a prevalence ranging from 0.35 % to 2.3 %, with a slight female predominance and a mean age at diagnosis of 57 years. Non‑functioning adenomas rarely need treatment and carry an excellent prognosis. Functioning adenomas, however, can cause Cushing syndrome, hyperaldosteronism, or virilization and often require medical or surgical intervention.

This adrenal lesion washout tool provides rapid diagnostic insight that can reduce unnecessary biopsies or follow‑up imaging. As with any decision‑support instrument, its results must be interpreted in the context of the full clinical and imaging picture, and it does not replace professional medical assessment.