Adrenal CT Washout Calculator

About the Adrenal CT Washout Calculator

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.

When This Page Helps

Adrenal washout only helps when the attenuation measurements are combined correctly and interpreted in the context of lesion size and hormone testing. This calculator keeps the APW and RPW steps together so the imaging values can be reviewed as a single characterization workflow instead of a few disconnected numbers.

How to Use the Inputs

1. Enter the unenhanced (pre-contrast) attenuation in Hounsfield Units (HU).
2. Enter the peak enhanced attenuation (60-90 seconds post-contrast).
3. Enter the delayed attenuation (15 minutes post-contrast).
4. Enter the lesion size in centimeters.
5. Select the laterality (left, right, or bilateral).
6. Review the APW, RPW, and classification result.
7. Use the differential table and washout curve for clinical context.

Example Calculation

Tips & Best Practices

• Place ROI consistently: use 2/3 of the lesion area, avoid edges, calcification, and necrosis.
• The delayed phase must be at 15 minutes — earlier delayed scans reduce washout values.
• Non-contrast CT is the first step: ≤10 HU = adenoma; <-20 HU = myelolipoma.
• Pheochromocytoma can show high washout (false positive) — always check biochemistry.
• MRI with chemical shift is the alternative when CT washout is equivocal.

The Adrenal Incidentaloma Workup

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.

Washout Protocol Technique

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.

When to Refer for Surgery

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.

Frequently Asked Questions

• What is the difference between APW and RPW?

  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.

• When is washout analysis not needed?

  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.

• What about lesions with 10-30 HU unenhanced?

  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.

• Does size matter in adrenal mass evaluation?

  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.

• What if washout is indeterminate?

  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. The next test depends on the clinical suspicion, hormone workup, and whether the lesion behaves like a lipid-poor adenoma or a non-adenoma lesion.

• Does this apply to bilateral adrenal masses?

  Bilateral masses have a different differential including metastases, lymphoma, hemorrhage, and bilateral adenomas. Each side should be evaluated independently with washout.