Adrenal CT Washout: Protocol, Formulas, Interpretation, and Pitfalls

Adrenal CT washout uses attenuation change between enhanced and delayed images to support characterization of selected adrenal lesions. Traditional criteria often use absolute percentage washout (APW) of at least 60% or relative percentage washout (RPW) of at least 40% on an appropriately timed protocol, but these values are not a stand-alone diagnosis of adenoma.

Clinical scope This guide is for radiology professionals. Apply current adrenal-incidentaloma guidance, lesion morphology, hormonal assessment, oncologic history, prior imaging, local protocol, and multidisciplinary judgment.

Before calculating

First decide whether washout is the right test. A homogeneous lesion measuring 10 HU or less on a technically adequate unenhanced CT is usually characterized without washout. A clearly myelolipomatous lesion, cyst, hemorrhage, or obviously aggressive mass should be handled according to its defining features rather than forced through the washout formulas.

Use truly unenhanced, enhanced, and delayed acquisitions obtained with compatible technique. Record delay time. Keep tube potential, reconstruction, slice selection, and region-of-interest placement as consistent as possible.

Formulas

Let U be unenhanced attenuation, E enhanced attenuation, and D delayed attenuation, all in HU.

• APW: ((E − D) / (E − U)) × 100
• RPW: ((E − D) / E) × 100

APW requires all three phases. RPW uses only enhanced and delayed values but still depends on a valid enhancement protocol. The adrenal washout calculator performs both calculations without changing the clinical prerequisites.

ROI technique

• Place a generously sized ROI within the lesion while avoiding necrosis, hemorrhage, calcification, fat, and partial volume with adjacent structures.
• Match the ROI location across phases using anatomic landmarks.
• If the lesion is heterogeneous, a single average may conceal clinically important components. Describe heterogeneity and sample deliberately.
• Recheck negative, implausibly high, or unstable results before reporting them.

Worked example

For U = 25 HU, E = 100 HU, and D = 50 HU:

• APW = ((100 − 50) / (100 − 25)) × 100 = 66.7%
• RPW = ((100 − 50) / 100) × 100 = 50%

These values meet traditional washout thresholds and may support a lipid-poor adenoma only when the protocol and lesion context are appropriate.

Important pitfalls

Pheochromocytoma, hypervascular metastasis, adrenocortical carcinoma, and other non-adenomas can overlap with adenoma washout values. Conversely, an adenoma may fail a threshold because of timing, technique, heterogeneity, or measurement variability. Washout failure does not equal malignancy, and threshold success does not exclude it.

Do not combine attenuation values from unrelated examinations or mismatched contrast phases. Avoid applying the formulas when enhancement is negligible because the denominator becomes unstable. For enlarging, heterogeneous, very large, invasive, or otherwise suspicious lesions, morphology and management guidance take priority over a reassuring percentage.

Report-ready wording

Homogeneous 1.8 cm left adrenal nodule measures 25 HU unenhanced, 100 HU enhanced, and 50 HU at the documented delayed phase. Calculated APW is 67% and RPW is 50%, meeting traditional washout criteria that can support a lipid-poor adenoma in the appropriate clinical setting. Correlate with current adrenal-incidentaloma guidance and biochemical assessment.

State the phase timing, values, formulas, technical limitations, comparison stability, and any suspicious morphology.

References

1. Fassnacht M, et al. European Society of Endocrinology clinical practice guideline on adrenal incidentalomas. Eur J Endocrinol. 2023.
2. Korobkin M, et al. Adrenal adenomas: relationship between histologic lipid and CT findings. Radiology. 1996.
3. Blake MA, et al. Adrenal imaging. Radiographics. Review of characterization techniques and pitfalls.