Adrenal Washout Calculator

Adrenal Washout Calculator for Endocrinology. The conventional diagnostic thresholds are APW at or above 60% and RPW at or above 40%, both measured at a 10-15 minute delay; values above these characterize a lipid-poor adenoma. In the original protocols a combination of unenhanced density 10 HU or less OR APW 60% or higher reached about 98% sensitivity and 92% specificity for adenoma. Washout at or below these cutoffs is indeterminate-to-malignant and should prompt further work-up such as chemical-shift MRI, biopsy, PET, or interval imaging. Because specificity is imperfect, a low washout is not proof of malignancy but a trigger for additional evaluation rather than an endpoint.

How this calculator works

This tool computes CT contrast washout for an indeterminate adrenal mass from three region-of-interest attenuation values: unenhanced (U), portal-venous/enhanced at ~60-90 seconds (E), and delayed at 10-15 minutes (D). Absolute percentage washout (APW) = (E - D) / (E - U) x 100, requiring an unenhanced series. Relative percentage washout (RPW) = (E - D) / E x 100, usable when no unenhanced scan exists. Both express how quickly contrast leaves the lesion; lipid-poor adenomas de-enhance rapidly while metastases, adrenocortical carcinoma, and pheochromocytoma retain contrast.

When to use this calculator

Apply this only to an adrenal mass that is indeterminate on unenhanced CT, meaning attenuation above 10 HU (lipid-poor), where a benign lipid-rich adenoma has already been excluded by the density measurement. It is most useful in oncology staging to separate an adenoma from a metastasis. Do NOT use washout to characterize markedly hyperattenuating lesions above roughly 43 HU on unenhanced CT, cystic or hemorrhagic masses, myelolipomas, or lesions with suspected pheochromocytoma, since pheochromocytomas frequently show adenoma-like washout and can mimic benign kinetics.

Inputs used

  • Noncontrast HU
  • Enhanced HU
  • Delayed HU

Clinical interpretation

The conventional diagnostic thresholds are APW at or above 60% and RPW at or above 40%, both measured at a 10-15 minute delay; values above these characterize a lipid-poor adenoma. In the original protocols a combination of unenhanced density 10 HU or less OR APW 60% or higher reached about 98% sensitivity and 92% specificity for adenoma. Washout at or below these cutoffs is indeterminate-to-malignant and should prompt further work-up such as chemical-shift MRI, biopsy, PET, or interval imaging. Because specificity is imperfect, a low washout is not proof of malignancy but a trigger for additional evaluation rather than an endpoint.

Worked example

A 3 cm right adrenal mass reads U = 22 HU (lipid-poor, indeterminate), E = 78 HU at 70 seconds, and D = 40 HU at 15 minutes. APW = (78 - 40) / (78 - 22) x 100 = 38/56 x 100 = 68%. RPW = (78 - 40) / 78 x 100 = 49%. APW of 68% exceeds the 60% threshold and RPW of 49% exceeds 40%, so the mass characterizes as a benign lipid-poor adenoma, and no biopsy or short-interval follow-up is needed.

Limitations and safety notes

Pheochromocytoma is the principal failure mode: many show avid washout above the adenoma thresholds, so biochemical exclusion is required before relying on kinetics. Reliability falls when the enhanced-phase attenuation is low (small numerator and denominator amplify measurement noise) or when timing deviates from the validated 60-90 second enhanced and 10-15 minute delayed phases, since scanner-, protocol-, and ROI-placement variation shift the numbers. The rules were derived largely in adenoma-versus-metastasis cohorts and perform less well for markedly hyperattenuating masses (unenhanced greater than ~43 HU), hypervascular metastases such as renal cell or hepatocellular carcinoma that can washout like adenomas, and collision tumors.

Frequently asked questions

What is the difference between absolute and relative washout?

Absolute percentage washout uses the true baseline unenhanced attenuation, so it needs a non-contrast series; relative percentage washout omits the unenhanced value and is used when only enhanced and delayed phases are available. APW threshold is 60% and RPW is 40%.

When should I NOT trust a high washout value?

When pheochromocytoma has not been biochemically excluded, when unenhanced density is above ~43 HU, and with hypervascular metastases (renal cell, HCC, melanoma), which can mimic adenoma washout. Confirm with biochemistry and consider MRI or biopsy.

What delay time should the delayed scan use?

The validated protocols use 10 to 15 minutes after contrast. Shorter delays reduce washout and can falsely place an adenoma below threshold; the 60% APW / 40% RPW cutoffs assume a 10-15 minute delayed acquisition.

Does washout replace unenhanced density measurement?

No. Washout is only needed when unenhanced attenuation is indeterminate (above 10 HU). A mass at or below 10 HU is already diagnostic of a lipid-rich adenoma and does not require contrast washout analysis.

Can this be used for adrenal masses in non-oncologic patients?

Yes, but the pretest probability of malignancy is lower, so an indeterminate washout in an incidentaloma more often reflects a benign lipid-poor adenoma or pheochromocytoma. Combine with size, growth, and hormonal work-up per incidentaloma guidelines.

References

  • Caoili EM, Korobkin M, Francis IR, Cohan RH, Platt JF, Dunnick NR, Raghupathi KI. Adrenal masses: characterization with combined unenhanced and delayed enhanced CT. Radiology. 2002. PMID: 11867777.
  • Pena CS, Boland GW, Hahn PF, Lee MJ, Mueller PR. Characterization of indeterminate (lipid-poor) adrenal masses: use of washout characteristics at contrast-enhanced CT. Radiology. 2000. PMID: 11110946.
  • Szolar DH, Korobkin M, Reittner P, Berghold A, Bauernhofer T, Trummer H, Schoellnast H, Preidler KW, Samonigg H. Adrenocortical carcinomas and adrenal pheochromocytomas: mass and enhancement loss evaluation at delayed contrast-enhanced CT. Radiology. 2005. PMID: 15671003.
  • Koo HJ, Choi HJ, Kim HJ, Kim SO, Cho KS. The value of 15-minute delayed contrast-enhanced CT to differentiate hyperattenuating adrenal masses compared with chemical shift MR imaging. Eur Radiol. 2014. PMID: 24647823.

Editorial review and citation methodology

Reviewed by the Quick Medical Calculator Editorial Team. Last reviewed: June 4, 2026. The review checks calculator inputs, intended population, interpretation, limitations, and source alignment.

  • Prefer original validation studies for scoring systems and prediction tools.
  • Use current specialty society guidance, transplant allocation policy, public health guidance, or regulator resources when they govern clinical use.
  • Include limitations and safety notes when a calculator is population-specific, context-dependent, or unsuitable as a standalone decision tool.

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