1. What is Absolute Percentage Washout?

Absolute Percentage Washout (APW) is a quantitative measurement used in CT characterization of adrenal nodules to differentiate between benign adenomas and malignant lesions based on their contrast enhancement and washout patterns.

2. How Does the Calculator Work?

The calculator uses the APW formula:

Where:

• \( Pre \) — Hounsfield Units pre-contrast
• \( PV \) — Hounsfield Units portal venous phase
• \( 15min \) — Hounsfield Units delayed phase (15 minutes)

Explanation: The formula calculates the percentage of contrast washout between portal venous and delayed phases relative to the baseline enhancement.

3. Importance of APW Calculation

Details: APW is crucial for characterizing adrenal nodules. Typically, APW ≥ 60% suggests benign adenoma, while APW < 60% raises suspicion for malignancy. This non-invasive method helps avoid unnecessary biopsies.

4. Using the Calculator

Tips: Enter Hounsfield Units from pre-contrast, portal venous, and 15-minute delayed CT scans. All values must be valid numerical measurements from the same adrenal nodule.

5. Frequently Asked Questions (FAQ)

Q1: What is the clinical significance of APW?
A: APW helps differentiate lipid-poor adrenal adenomas from metastases, with ≥60% washout strongly favoring benign adenoma.

Q2: How does APW differ from Relative Percentage Washout?
A: APW uses pre-contrast values in the denominator, while RPW uses portal venous values. APW is generally more specific for adenoma diagnosis.

Q3: What are typical APW values for adrenal lesions?
A: Adenomas typically show APW ≥60%, metastases usually <60%, and pheochromocytomas often show rapid washout but variable patterns.

Q4: Are there limitations to APW calculation?
A: Accuracy depends on proper timing of contrast phases and consistent ROI placement. May be less reliable in very small nodules or heterogeneous lesions.

Q5: When should adrenal washout CT be performed?
A: Recommended for characterizing indeterminate adrenal nodules >1 cm discovered incidentally on CT, especially in oncology patients.