Adrenal Washout Calculator

Adrenal adenomas typically show rapid uptake and washout of intravenous contrast media compared to malignant lesions or lipid-poor tumors. This calculator evaluates attenuation values measured in Hounsfield Units (HU) across three crucial phases: precontrast (unenhanced), postcontrast (portal venous phase), and delayed (usually 10 to 15 minutes post-injection). The absolute washout formula measures the percentage loss of enhancement relative to the peak postcontrast value: Absolute Washout = 100 × ((Postcontrast - Delayed) / (Postcontrast - Precontrast)). The relative washout formula is used when an unenhanced scan is unavailable: Relative Washout = 100 × ((Postcontrast - Delayed) / Postcontrast).

Consider an incidental adrenal mass discovered during a routine abdominal CT scan. The radiologist measures the attenuation values in Hounsfield Units as follows: Precontrast attenuation is 10 HU, Postcontrast attenuation is 110 HU, and Delayed attenuation (at 15 minutes) is 40 HU. To find the absolute washout, substitute these numbers into the formula: Absolute Washout = 100 × ((110 - 40) / (110 - 10)). This simplifies to 100 × (70 / 100), which yields 70 percent. Since an absolute washout greater than 60 percent strongly indicates a benign adenoma, this result helps avoid unnecessary invasive biopsies.

Ensure that region-of-interest (ROI) measurements are placed consistently over the same area of the adrenal mass on all phases, avoiding calcifications, cystic areas, or surrounding retroperitoneal fat to prevent skewed Hounsfield Unit readings. Always verify that the delay time aligns strictly with the imaging protocol used to derive validation thresholds, typically between 10 and 15 minutes for delayed scans. When unenhanced imaging is missing, rely on the relative washout formula, keeping in mind that the diagnostic cutoff for a benign lesion in relative washout is typically set at 40 percent or higher.