Calculate absolute and relative percentage washout for adrenal masses. Differentiates adenoma from metastasis/carcinoma with contrast enhancement curves.
The Adrenal CT Washout Calculator computes both the absolute percentage washout (APW) and relative percentage washout (RPW) from contrast-enhanced CT attenuation values to help characterize adrenal masses as benign adenomas versus potentially malignant lesions.
Adrenal incidentalomas are found in approximately 4-5% of abdominal CT scans. Most are benign adenomas, but differentiating them from metastases, pheochromocytomas, and adrenocortical carcinomas is critical. The washout technique exploits the fact that adenomas have a rich capillary network that loses contrast quickly (high washout), while malignant lesions retain contrast due to neovascularity and impaired capillary permeability (low washout).
The calculator uses three CT attenuation measurements: unenhanced (pre-contrast), peak enhanced (60-90 seconds), and delayed (15 minutes). An APW ≥60% or RPW ≥40% has high sensitivity and specificity for adenoma diagnosis. Lesions with unenhanced attenuation ≤10 HU are considered lipid-rich adenomas and don't require washout analysis. The tool also flags size-based warnings (≥4 cm raises concern for adrenocortical carcinoma) and provides a comprehensive differential diagnosis table.
The adrenal washout calculation requires specific formulas that are easy to miscalculate under clinical pressure. This calculator ensures accuracy, provides immediate classification, and includes the differential diagnosis context needed for clinical decision-making. Keep these notes focused on your operational context. Tie the context to the calculator’s intended domain. Use this clarification to avoid ambiguous interpretation.
APW = ((Enhanced − Delayed) / (Enhanced − Unenhanced)) × 100 RPW = ((Enhanced − Delayed) / Enhanced) × 100 Adenoma criteria: APW ≥ 60% or RPW ≥ 40% Lipid-rich adenoma: Unenhanced ≤ 10 HU (no washout needed)
Result: APW = 76.5%, RPW = 62.5% — Likely adenoma (washout criteria met)
Both APW (76.5% ≥ 60%) and RPW (62.5% ≥ 40%) meet adenoma criteria. The unenhanced value of 22 HU indicates a lipid-poor adenoma that required washout analysis for characterization.
When an adrenal mass is discovered incidentally, the two key questions are: (1) Is it functioning (producing excess hormones)? and (2) Is it malignant? Biochemical workup (cortisol, catecholamines, aldosterone) addresses the first question. CT washout analysis, along with unenhanced attenuation and size, addresses the second.
The standard protocol requires three CT acquisitions: unenhanced, enhanced (60-90 seconds after IV contrast), and delayed (15 minutes). ROI measurement should be consistent across all phases. The calculation assumes uniform lesion enhancement and washout — heterogeneous lesions with necrosis or hemorrhage may give unreliable results.
Surgical referral is recommended for: functioning tumors (after appropriate medical optimization), lesions ≥4 cm, lesions with rapid growth on follow-up (>1 cm/year), and indeterminate lesions with high FDG avidity on PET/CT. The choice between laparoscopic adrenalectomy and open surgery depends on size and malignancy suspicion.
APW requires all three phases (unenhanced, enhanced, delayed) and is more accurate. RPW can be calculated without the unenhanced phase using only enhanced and delayed values. APW ≥60% or RPW ≥40% suggests adenoma.
If unenhanced attenuation is ≤10 HU, the lesion is a lipid-rich adenoma with high confidence (sensitivity ~71%, specificity ~98%). No contrast or washout is needed.
These are "lipid-poor" adenomas or other entities. Washout analysis is essential for this group. About 30% of adenomas are lipid-poor and require the full washout protocol.
Yes. Adrenocortical carcinoma risk increases with size: <4 cm (2%), 4-6 cm (6%), >6 cm (25%). Lesions ≥4 cm are often surgically excised regardless of imaging characteristics.
Options include chemical shift MRI (detects intracellular lipid), PET/CT (high FDG uptake suggests malignancy), or interval follow-up CT at 6-12 months to assess growth. Use this as a practical reminder before finalizing the result.
Bilateral masses have a different differential including metastases, lymphoma, hemorrhage, and bilateral adenomas. Each side should be evaluated independently with washout.