Adrenal Washout Calculator: Adenoma vs Non-Adenoma CT
Enter the Hounsfield Unit (HU) values from a three-phase adrenal CT protocol to calculate absolute washout percentage (AWP) and relative washout percentage (RWP). The calculator instantly interprets whether the nodule is consistent with a benign adrenal adenoma or requires further workup. If the pre-contrast scan was not obtained, the relative washout formula uses only the portal venous and delayed phases. This tool is for educational and informational use; all findings should be interpreted by a qualified radiologist or clinician in the full clinical context.
Formula
Worked example
A 2 cm adrenal nodule: pre-contrast 30 HU, portal venous 100 HU, delayed 55 HU. AWP = 100 x (100 - 55) / (100 - 30) = 100 x 45 / 70 = 64.3% (meets adenoma threshold >= 60%). RWP = 100 x (100 - 55) / 100 = 45% (meets relative threshold >= 40%). Both washout values support a benign adrenal adenoma.
What is adrenal washout and why does it matter?
An adrenal "incidentaloma" is an adrenal nodule found by chance on cross-sectional imaging performed for another reason. Most are benign cortical adenomas, but the differential includes pheochromocytoma, metastasis, adrenocortical carcinoma, and myelolipoma. Because biopsy carries risks and is often non-diagnostic, non-invasive CT characterization has become the first-line approach. The key physiological principle is that adrenal adenomas contain abundant intracytoplasmic lipid droplets. These cause two measurable CT features: low pre-contrast attenuation and rapid contrast washout. Malignant lesions and most metastases lack this lipid content and retain contrast longer, producing slower washout curves. The adrenal CT washout protocol exploits this difference by measuring attenuation at three time points: before contrast, at 60-75 seconds (portal venous phase), and at 15 minutes (delayed phase).
The two washout formulas explained
Absolute Washout Percentage (AWP) uses all three phases:
AWP = 100 x (portal venous HU - delayed HU) / (portal venous HU - pre-contrast HU)
The denominator represents the total contrast enhancement; the numerator is how much of that enhancement was lost by 15 minutes. A value of 60% or above is consistent with a benign adenoma.
Relative Washout Percentage (RWP) needs only the enhanced phases:
RWP = 100 x (portal venous HU - delayed HU) / portal venous HU
This is useful when a pre-contrast phase was not obtained or is unavailable. A value of 40% or above suggests adenoma.
Both formulas were validated in the landmark study by Caoili et al. (Radiology, 2002) on 61 lipid-poor adrenal masses (pre-contrast HU >= 10). AWP achieved 86% sensitivity and 92% specificity; RWP achieved 82% sensitivity and 92% specificity.
How to use this calculator
Enter the mean attenuation values measured on a region of interest (ROI) over the adrenal nodule at each phase. Use the same ROI placement and size at each time point for consistency, placing the ROI within the soft tissue component and avoiding areas of calcification, cyst, or hemorrhage.
- Pre-contrast HU: measured on the unenhanced series. If the pre-contrast phase was not acquired, leave the default or note that only relative washout will be meaningful.
- Portal venous phase HU: measured at 60-75 seconds post-injection.
- Delayed phase HU: measured 15 minutes post-injection.
The calculator returns both AWP and RWP with interpretation, and flags the pre-contrast attenuation against the 10 HU lipid-rich threshold and the 43 HU suspicion threshold.
Important caveats and limitations
Adrenal CT washout criteria are not universally applicable. Do not use washout criteria in the following situations:
- Known hypervascular primary tumor (renal cell carcinoma, hepatocellular carcinoma, melanoma, carcinoid): metastases from these primaries can wash out rapidly, mimicking adenoma and reducing specificity.
- Heterogeneous lesions with necrosis, hemorrhage, or calcification: the attenuation is not representative of the lesion as a whole.
- Large adenomas (>= 3 cm): sensitivity falls to approximately 67% in some series; size alone does not disqualify washout, but confidence is lower.
- Pre-contrast HU > 43: even with high washout, malignancy is not excluded.
- Pheochromocytoma: roughly 30-45% of pheos overlap with lipid-poor adenoma washout patterns, so biochemical workup (plasma or urine metanephrines) is essential before attributing any adrenal mass to adenoma.
This calculator is for educational and informational purposes. All CT characterizations must be interpreted by a qualified radiologist in the clinical context.
CT adrenal washout interpretation thresholds
| Metric | Threshold | Interpretation | Sensitivity | Specificity |
|---|---|---|---|---|
| Pre-contrast HU | <= 10 HU | Lipid-rich adenoma: diagnosis without washout | 71% | 98% |
| Absolute Washout (AWP) | >= 60% | Consistent with adenoma | 86% | 92% |
| Relative Washout (RWP) | >= 40% | Consistent with adenoma (no pre-contrast required) | 82% | 92% |
| Pre-contrast HU | > 43 HU | Suspicious: washout does not exclude malignancy | - | - |
Validated thresholds from Caoili et al. (Radiology 2002) for lipid-poor adenomas (pre-contrast HU >= 10). Sensitivity and specificity apply to that cohort.
Frequently asked questions
What is the adrenal washout threshold for adenoma?
The widely accepted thresholds are an absolute washout percentage (AWP) of 60% or above, or a relative washout percentage (RWP) of 40% or above. Both are strongly suggestive of a benign adrenal adenoma. These cutoffs come from the Caoili et al. Radiology 2002 study and have been replicated in multiple subsequent series.
Do I need a pre-contrast CT phase for adrenal washout?
Not always. If the pre-contrast phase was obtained and the nodule measures 10 HU or below, the lesion is a lipid-rich adenoma and washout is not needed for diagnosis. If the pre-contrast attenuation is above 10 HU, washout criteria apply. If no pre-contrast phase is available, the relative washout formula (RWP) uses only the portal venous and 15-minute delayed phases. AWP requires all three phases.
What does a lipid-rich adenoma mean on CT?
A lipid-rich adenoma has a pre-contrast attenuation of 10 HU or below on unenhanced CT. The intracytoplasmic lipid lowers the density below surrounding soft tissue. This finding is essentially diagnostic of a benign adenoma with about 71% sensitivity and 98% specificity, so contrast-enhanced washout imaging may not be necessary.
Can a pheochromocytoma look like an adenoma on washout imaging?
Yes. Approximately 30-45% of pheochromocytomas have washout values that overlap with the adenoma range. This is why biochemical screening for pheochromocytoma (plasma free metanephrines or 24-hour urine metanephrines) should be performed in all patients with an adrenal incidentaloma before any intervention, regardless of CT washout results.
Why is relative washout lower than absolute washout for the same nodule?
Because the denominators differ. AWP denominates by the enhancement above baseline (post minus pre), while RWP denominates by the total post-contrast attenuation (post HU alone, which includes the baseline). When the pre-contrast attenuation is substantial, AWP amplifies the washout fraction relative to RWP. That is why the adenoma threshold for AWP (60%) is higher than for RWP (40%).
Should I use washout criteria for a large adrenal nodule?
With caution. Studies have shown that sensitivity for adenoma detection drops to around 67% in nodules 3 cm or larger. The criteria still have value but are less reliable. Large or heterogeneous lesions should be evaluated with additional imaging (MRI chemical shift, PET-CT) or histology depending on clinical context.