Adrenal Washout Calculator
Adrenal Washout Calculator - CT Absolute & Relative Percentage Washout
Free adrenal CT washout calculator for radiologists. Calculate absolute and relative percentage washout (APW, RPW) in Hounsfield units to characterize adrenal nodules and incidentalomas. Includes clinical interpretation, report text generator, and sensitivity/specificity reference based on ACR and ESE guidelines.
Attenuation (Hounsfield units)
Report text options (not used in calculation)
Washout values
Interpretation
Enter enhanced and delayed HU to see interpretation.
Warnings & caveats
Sensitivity & specificity reference ▼
Unenhanced CT ≤10 HU for adenoma: Sensitivity 71%, Specificity 98% (Boland et al., AJR 1998).
APW ≥60% for adenoma: Sensitivity 86–88%, Specificity 92–96% (Caoili et al., Radiology 2002; Blake et al., Radiology 2006).
RPW ≥40% for adenoma: Sensitivity 82–83%, Specificity 92–93% (Caoili et al., Radiology 2002).
When to use adrenal washout
Use this adrenal washout calculator when you have an incidental adrenal nodule that measures >10 HU on unenhanced CT (or when no unenhanced series is available). Enter attenuation values in Hounsfield units (HU) for each phase; the tool computes absolute percentage washout (APW) and relative percentage washout (RPW) to help distinguish benign adrenal adenoma and lipid-poor adenoma from lesions that need further workup.
The protocol
A standard adrenal washout CT includes: (1) unenhanced scan, (2) portal venous phase at 60–70 seconds after IV contrast, and (3) delayed phase at 10–15 minutes. Place the ROI on the same region of the lesion across all phases for reliable APW and RPW.
Interpreting the results
Pre-contrast ≤10 HU is diagnostic of lipid-rich adenoma. When pre-contrast is 11–43 HU or unavailable, use washout: APW ≥60% or RPW ≥40% (when no unenhanced scan) suggests benign adenoma. Below these thresholds, the finding is indeterminate. Pre-contrast >43 HU raises concern for malignancy regardless of washout.
Pitfalls
Pheochromocytomas can show washout mimicking adenoma; lesions enhancing to >120 HU should not be called adenoma. In patients with known hypervascular primaries (e.g. RCC, HCC), washout has reduced specificity. Sensitivity decreases for larger adenomas (≥3 cm) and heterogeneous lesions.
When washout isn’t enough
For indeterminate lesions, MRI with chemical shift imaging is the usual next step. Biopsy or PET/CT may be appropriate depending on clinical context and guidelines (ACR, ESE).
References
- Mayo-Smith WW, et al. Management of Incidental Adrenal Masses: A White Paper of the ACR Incidental Findings Committee. J Am Coll Radiol. 2017;14:1038-1044. PubMed
- Fassnacht M, et al. European Society of Endocrinology clinical practice guidelines on the management of adrenal incidentalomas. Eur J Endocrinol. 2023;189(1):G1-G42. PubMed
- Caoili EM, et al. Adrenal masses: characterization with combined unenhanced and delayed enhanced CT. Radiology. 2002;222(3):629-633. PubMed
- Blake MA, et al. Distinguishing benign from malignant adrenal masses: multi-detector row CT protocol with 10-minute delay. Radiology. 2006;238(2):578-585. PubMed
- Boland GW, et al. Characterization of adrenal masses using unenhanced CT: an analysis of the CT literature. AJR. 1998;171(1):201-204. PubMed
- Schieda N, Siegelman ES. Update on CT and MRI of Adrenal Nodules. AJR. 2017;208(6):1206-1217. AJR
- Patel J, et al. Can established CT attenuation and washout criteria for adrenal adenoma accurately exclude pheochromocytoma? AJR. 2013;201(1):122-127. AJR