CT Adrenal Washout Calculator
Use attenuation values (HU) from adrenal CT phases to calculate absolute and relative washout.
APW = ((Enhanced − Delayed) / (Enhanced − Unenhanced)) × 100
RPW = ((Enhanced − Delayed) / Enhanced) × 100
What is adrenal washout?
Adrenal washout is a CT-based method used to characterize adrenal masses, especially incidental adrenal nodules found during imaging for unrelated reasons. The central idea is simple: some lesions take up contrast and then release it quickly, while others retain contrast longer. Measuring how quickly attenuation drops between enhanced and delayed imaging can help estimate whether a lesion behaves like a benign adenoma.
In practical radiology workflows, two metrics are commonly used:
- Absolute Percentage Washout (APW) when unenhanced, enhanced, and delayed values are all available.
- Relative Percentage Washout (RPW) when unenhanced data is missing but enhanced and delayed values are available.
How to use this calculator
Step 1: Gather HU values from your CT report
- Unenhanced attenuation (if available)
- Enhanced attenuation
- Delayed attenuation (usually 10-15 minutes)
Step 2: Enter values
Enter the values exactly as reported in Hounsfield Units (HU). If your report does not include a non-contrast value, leave that field blank and the calculator will compute RPW only.
Step 3: Review interpretation
The output provides APW and/or RPW plus a plain-language interpretation based on common threshold values used in adrenal imaging literature.
Common threshold guide
| Metric | Typical Benign Adenoma Cutoff | Meaning |
|---|---|---|
| APW | ≥ 60% | Supports adenoma-like washout pattern |
| RPW | ≥ 40% | Supports adenoma-like washout pattern |
Worked example
Suppose an adrenal lesion has:
- Unenhanced = 18 HU
- Enhanced = 92 HU
- Delayed = 34 HU
Then:
- APW = ((92 - 34) / (92 - 18)) × 100 = 78.4%
- RPW = ((92 - 34) / 92) × 100 = 63.0%
Both values exceed common thresholds, which is generally compatible with an adenoma-type washout profile.
Clinical context still matters
Washout values are one piece of the puzzle. Radiologists and endocrinologists also consider lesion size, growth over time, lipid-rich appearance on unenhanced CT, MRI signal behavior, hormonal workup, and patient history (including known malignancy).
A lesion can occasionally show atypical behavior due to timing differences, measurement variability, or mixed tissue composition. That is why interpretation should be integrated with the complete report and clinical findings.
Measurement tips to reduce error
- Use consistent ROI placement across phases.
- Avoid necrotic, calcified, cystic, or hemorrhagic portions.
- Use protocol-consistent delayed timing (commonly around 10-15 minutes).
- Confirm phase labels to avoid swapping enhanced and delayed values.
FAQ
Can I use RPW alone?
Yes, RPW is commonly used when non-contrast values are unavailable. APW is preferred when all three phases are present.
What if my washout value is negative?
Negative washout usually means delayed attenuation is higher than enhanced attenuation, which can happen with input errors, protocol differences, or unusual lesion kinetics. Recheck values and clinical imaging details.
Does this diagnose cancer?
No. This calculator supports lesion characterization but does not diagnose malignancy. Final interpretation belongs to qualified clinicians using full imaging and clinical context.