Adrenal washout calculator (CT)

Note: this calculator should be used primarily for adrenal washout protocol multi-phase CT examinations of lipid-poor adrenal lesions. Please also note that >4 cm lesions are concerning for malignancy regardless of their washout characteristics. For further information please read the detailed notes below the calculator.

Notes

The washout formulas

• Absolute washout:
  • [(HU_{portal venous phase}) – (HU_delayed)] / [(HU_{portal venous phase}) – (HU_non-enhanced)] x 100
• Relative washout:
  • [(HU_{portal venous phase}) – (HU_delayed)] / (HU_{portal venous phase}) x 100

Practical points and limitations of washout imaging

• On non-contrast imaging
  • an adrenal lesion with a HU <0 HU can be reported as a lipid-rich adenoma (47% sensitivity, 100% specificity)
  • for lesions <10 HU the probability is still very high (71% sensitivity, 98% specificity).
  • ^{Blake MA, Holalkere NS, Boland GW. Imaging techniques for adrenal lesion characterization. Radiol Clin North Am. 2008 Jan;46(1):65-78, vi. doi: 10.1016/j.rcl.2008.01.003. PMID: 18328880.}
• Adrenal lesions with a nonenhanced density >43 HU that are neither calcified, nor hemorrhagic should be treated as suspicious for malignancy. In these cases FDG-PET/CT and other diagnostic strategies should be considered instead of washout imaging.
  • ^{Blake MA, Kalra MK, Sweeney AT, Lucey BC, Maher MM, Sahani DV, Halpern EF, Mueller PR, Hahn PF, Boland GW. Distinguishing benign from malignant adrenal masses: multi-detector row CT protocol with 10-minute delay. Radiology. 2006 Feb;238(2):578-85. doi: 10.1148/radiol.2382041514. Epub 2005 Dec 21. PMID: 16371582.}
• Whenever possible the absolute washout should be preferred during decision making. The relative washout formula is a problem solving tool for those situations where an incidental adrenal lesion is detected during an contrast-enhanced abdominal CT exam where no non-contrast acquisition was performed.
• Pheochromocytomas have overlapping washout characteristics with lipid-poor adenomas. An indeterminate adrenal lesion with peak enhancement >130 HU is concerning for hypervascular pheochromocytoma.
  • ^{Northcutt BG, Trakhtenbroit MA, Gomez EN, Fishman EK, Johnson PT. Adrenal Adenoma and Pheochromocytoma: Comparison of Multidetector CT Venous Enhancement Levels and Washout Characteristics. J Comput Assist Tomogr. 2016 Mar-Apr;40(2):194-200. doi: 10.1097/RCT.0000000000000343. PMID: 26978001.}
• Also in a clinical setting where hypervascular metastases (e.g. renal cell or hepatocellular carcinoma) are suspected washout has again a limited value.
• The pretest probability of malignancy sharply increases, and becomes 70% in lesions >4 cm, further increasing to 85% in lesions >6 cm.
• Ideally the region of interest (ROI) should cover approximately two-third of the lesion, without including the periphery to reduce partial volume averaging effects of the adjacent fat.
  • ^{Nandra G, Duxbury O, Patel P, Patel JH, Patel N, Vlahos I. Technical and Interpretive Pitfalls in Adrenal Imaging. Radiographics. 2020 Jul-Aug;40(4):1041-1060. doi: 10.1148/rg.2020190080. PMID: 32609593.}
• In case of a single phase contrast-enhanced CT a small adrenal lesion with less than or equal to 10 HU density has been reported as an acceptable cutoff to diagnose adrenal adenoma, though with limited sensitivity.

For further reference please visit the Radiopaedia.org articles on adrenal washout and adrenal adenomas.

Last updated: 2021-10-24