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Adrenal Washout Calculator (CT Scan)

Calculate Absolute and Relative Washout percentages for adrenal incidentalomas. Differentiates benign adenomas from metastases based on CT Hounsfield Units.

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About

When an adrenal mass is discovered incidentally on a CT scan, differentiating a benign lipid-poor adenoma from a malignant metastasis is critical. The Washout Calculator utilizes the physiological property that adenomas take up and wash out contrast medium faster than non-adenomas.

This tool computes both Absolute Washout (if a non-contrast scan is available) and Relative Washout. Precise Hounsfield Unit (HU) inputs from Non-contrast, Venous (Portal), and Delayed phases are required for accurate characterization.

radiology CT scan adrenal washout adenoma incidentaloma

Formulas

Calculations depend on Hounsfield Units (HU) measured at different time points.

Absolute Washout (APW):

HUvenous − HUdelayedHUvenous − HUnative × 100

Relative Washout (RPW):

HUvenous − HUdelayedHUvenous × 100

Reference Data

MetricThresholdInterpretation
Absolute Washout> 60%Consistent with Benign Adenoma.
Absolute Washout< 60%Indeterminate / Possible Malignancy.
Relative Washout> 40%Consistent with Benign Adenoma.
Relative Washout< 40%Indeterminate / Possible Malignancy.

Frequently Asked Questions

You can only calculate Relative Washout (RPW). However, Absolute Washout is generally considered more specific. RPW relies only on the enhanced and delayed phases.
Standard protocol typically requires a delay of 15 minutes after contrast injection. Scanning too early (e.g., 5 minutes) can lead to false-negative washout results.
No. Pheochromocytomas and some hypervascular metastases can mimic the washout kinetics of adenomas. This calculator is primarily for distinguishing adenomas from metastases in patients with known primaries (e.g., lung cancer).