The formula used for the above calculator is:

If the relative washout is greater than 40% then this classifies the lesion as a benign adenoma.

Background

If an adrenal lesion has average, unenhanced Hounsfield units of 10 or less then it can be assumed that in most cases the diagnosis is a lipid rich adenoma. If the average Hounsfield unit is less than 0, the specificty is 100%(1,2).

Where a lesion cannot be characterised based on the unenhanced Hounsfield unit values, post contrast (at 60 seconds) and delayed (15 minute) scans are required. Using Hounsfield unit measurements from these scans, either the relative or absolute contrast washout can be calculated and used to characterise the lesion.(3,4) Use the calculator above for relative washout when a pre-contrast scan is not available. When a pre-contrast scan is available, use the absolute washout calculator.

Further reading

1. A. Sahdev, J. Willatt, I. R. Francis, and R. H. Reznek, The indeterminate adrenal lesion, Cancer Imaging, vol. 10, no. 1, pp. 102-113, Mar. 2010.

References

1. Lee MJ, Hahn PF, Papanicolaou N, et al. Benign and malignant adrenal masses: CT distinction with attenuation coefficients, size, and observer analysis. Radiology. 1991;179(2):415-418.
2. Boland GW, Lee MJ, Gazelle GS, et al. Characterization of adrenal masses using unenhanced CT: an analysis of the CT literature. American Journal of Roentgenology. 1998;171(1):201-204.
3. Caoili EM, Korobkin M, Francis IR, et al. Adrenal Masses: Characterization with Combined Unenhanced and Delayed Enhanced CT1. Radiology. 2002;222(3):629-633.
4. Korobkin M, Brodeur FJ, Francis IR, et al. CT time-attenuation washout curves of adrenal adenomas and nonadenomas. American Journal of Roentgenology. 1998;170(3):747-752.