Adrenal incidentaloma — CT / MRI

CTMRIACR incidental adrenal mass (2017)

Incidentally detected adrenal nodule characterized on CT or MRI.

Applies to

Incidentally detected adrenal mass on CT or MRI

Does not apply to

Known extra-adrenal malignancy with heterogeneous or growing lesions (favor further workup)
Suspected pheochromocytoma or adrenocortical carcinoma — do not rely on washout

What to report

Tap an item to check it off while dictating.

1.Size and laterality
Maximum diameter and side; note bilaterality.
2.Unenhanced attenuation
Mean HU through the largest homogeneous portion. ≤10 HU indicates a lipid-rich adenoma.
Pitfall: Place the ROI over at least two-thirds of the lesion, avoiding necrosis and calcification.
3.Homogeneity and margins
Homogeneous with smooth margins favors adenoma; heterogeneity or irregular margins is concerning.
4.Washout (if indeterminate)
For homogeneous 1–4 cm masses >10 HU, obtain 60–75 s and 15 min delayed phases and compute washout.
5.Chemical-shift MRI
On in-/opposed-phase MRI, signal drop on opposed-phase indicates intracellular lipid (adenoma). Quantify with the signal intensity index.
6.Macroscopic fat
Macroscopic fat indicates myelolipoma.
7.Comparison / stability
Stability ≥12 months supports a benign etiology; interval growth warrants further evaluation.

Scores & calculators

Adrenal CT washout

ACR incidental adrenal mass (2017)

Absolute and relative washout to characterize a homogeneous adrenal mass.

Unenhanced attenuation

Enhanced (60–75 s post-contrast)

Delayed (15 min)

Complete the inputs to see the result.

Absolute washout (APW) = (enhanced − delayed) / (enhanced − unenhanced) × 100; ≥60% = adenoma and is the primary metric when an unenhanced phase is available. Relative washout (RPW) = (enhanced − delayed) / enhanced × 100; ≥40% is a supportive criterion, most useful when no unenhanced phase was obtained. Unenhanced ≤10 HU alone indicates a lipid-rich adenoma (washout unnecessary). Apply only to homogeneous masses; do not use when pheochromocytoma, hypervascular metastasis, or adrenocortical carcinoma is suspected.

Adrenal chemical-shift signal index

Signal intensity index for a lipid-rich adenoma on in-/opposed-phase MRI.

In-phase signal

Opposed-phase signal

Complete the inputs to see the result.

Signal intensity index = (in-phase − opposed-phase) / in-phase × 100. A drop ≥ 16.5% at 1.5 T indicates a lipid-rich adenoma. Note: clear-cell RCC and HCC metastases can also drop signal; the spleen is an unreliable internal reference in iron overload (use kidney).

Sources

Incidental adrenal mass white paper — Mayo-Smith WW et al. J Am Coll Radiol (2017)
Adrenal washout CT — Caoili EM et al. Radiology (2002)

Content review: Content pending clinical review · 2026-07-09

Educational reference for trained clinicians. Verify every result against the cited original guideline before use. Not a substitute for clinical judgment or medical advice.