How is Absolute Percentage Washout (APW) calculated?

APW is calculated by subtracting the delayed HU from the enhanced HU, and dividing that result by the difference between the enhanced HU and unenhanced HU. Multiply by 100 to get the percentage.

What does a high adrenal washout mean?

A high adrenal washout (APW > 60% or RPW > 40%) strongly suggests that the adrenal mass is a benign adenoma rather than a metastasis or other malignant tumor.

What if I do not have an unenhanced CT scan?

If an unenhanced scan was not performed, you can still evaluate the mass using the Relative Percentage Washout (RPW) formula, which relies solely on the enhanced and 15-minute delayed scans.

Adrenal Washout Calculator

CT Scan Data

Unenhanced Phase (HU)

Enhanced Phase (60-70s) (HU)

Delayed Phase (15m) (HU)

Washout Results

Clinical Evaluation

Enter Scan Values

Absolute Washout (APW)

--%

Relative Washout (RPW)

--%

An Adrenal Washout Calculator is a specialized medical imaging tool utilized by radiologists to compute the clearance rate of contrast material in adrenal nodules. By analyzing the rapid inflow and outflow of contrast dye on a CT scan, medical professionals can effectively differentiate between a benign adrenal adenoma and a malignant or non-adenoma lesion without invasive biopsies.

Understanding the Washout Formulas

Adrenal adenomas have highly specific vascular characteristics, meaning they absorb contrast dye quickly and wash it out just as fast. Non-adenomas hold onto the dye for much longer. The calculator uses two standardized equations based on Hounsfield Units (HU):

Absolute Percentage Washout (APW): Evaluates all three phases. Formula: ((Enhanced HU - Delayed HU) / (Enhanced HU - Unenhanced HU)) × 100.
Relative Percentage Washout (RPW): Evaluates only the contrast phases. Formula: ((Enhanced HU - Delayed HU) / Enhanced HU) × 100.

How to Use This Radiological Tool

Input the Unenhanced Phase attenuation in Hounsfield Units. If you do not have a pre-contrast scan, leave this field empty (the tool will calculate RPW automatically).
Input the Enhanced Phase attenuation. This is typically measured 60 to 70 seconds post-contrast injection.
Input the Delayed Phase attenuation, which is measured 15 minutes after the initial contrast injection.
Review the Clinical Evaluation. The tool instantly flags the mass as a likely benign adenoma, a non-adenoma, or an indeterminate mass based on standard clinical thresholds.

Frequently Asked Questions

What is the diagnostic threshold for a benign adenoma?

According to widely accepted radiological standards, an Absolute Percentage Washout (APW) of 60 percent or higher is diagnostic of a benign adrenal adenoma. Alternatively, if an unenhanced scan is unavailable, a Relative Percentage Washout (RPW) of 40 percent or higher indicates an adenoma.

Why is the 15-minute delay crucial?

The 15-minute delayed phase provides the exact time required to observe the rapid contrast de-enhancement that is characteristic of adenomas. Scanning too early or too late can severely skew the calculation and lead to an indeterminate or incorrect assessment.

Can this tool replace a radiologist's diagnosis?

No. This adrenal washout calculator is strictly an educational and supportive computational tool. Clinical decisions must always account for patient history, previous imaging comparisons, lesion morphology, and the professional judgment of a board-certified physician or radiologist.