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Adrenal Nodule Evaluation

Quantitative characterization of an adrenal nodule. Fill any sections that apply; each gives an interpretation and feeds a copy-to-report.

→ ACR Adrenal Incidentaloma (management algorithm)

1 · Non-contrast CT (first-line)

Unenhanced attenuationHU

2 · Adrenal washout CT

unenhanced · portal venous (60–75 s) · delayed (10–15 min)

Unenhanced HU (optional)HU

Enhanced (portal venous) HUHU

Delayed HUHU

3 · Chemical-shift MRI

Field strength

SI in-phase

SI opposed-phase

T2 SI ratio (lesion : spleen/muscle)

Adrenal-to-spleen ratio (optional)

Adrenal IP / OP

Spleen IP / OP

ASR uses the spleen as an internal reference to normalize the adrenal signal drop. It performs comparably to the simpler adrenal-only index (SII = (IP−OP)/IP×100) — both give sensitivity ~89–93% and specificity ~90%, and qualitative visual assessment of signal drop is often as good. ASR adds nothing over SII when the drop is obvious; its spleen normalization can be confounded by splenic iron, and — like all chemical-shift methods — it fails for lipid-poor adenomas and when non-contrast CT is >30 HU (little microscopic lipid). Use only for homogeneous lesions; prefer non-contrast CT ≤10 HU first.

4 · Size

Maximal diametercm

Interval growth (optional)

Prior diametercm

Intervalmonths

5 · Dual-energy CT metrics (advanced)

Enter any DECT values you have — each metric computes independently, so one or two fields is enough. VNC (virtual non-contrast HU) is the shared denominator: include it to get the iodine ratio and RER. Outputs: fat fraction (≥23.8% specific for adenoma), iodine:VNC ratio = iodine ÷ VNC (≥6.7 favors adenoma), and RER = relative enhancement ratio = portal-venous HU ÷ VNC × 100 (≥214% specific for adenoma).

VNC attenuationHU

Iodine densitymg/mL

Portal venous HU (for RER)HU

Fat fraction%

6 · FDG-PET/CT

tumor : liver SUVmax ratio

Tumor SUVmax

Liver SUVmax

Suggested pathway (decision tree)

Non-contrast CT

Copy to report

Fill any section above to build the report.

Thresholds & references

Non-contrast CT

≤10 HU = lipid-rich adenoma (near-100% specific). >10 HU = indeterminate → washout CT or chemical-shift MRI. Markedly negative HU / macroscopic fat → myelolipoma.

Washout

APW = (enhanced − delayed)/(enhanced − unenhanced) × 100; adenoma if >60%. RPW = (enhanced − delayed)/enhanced × 100; adenoma if >40%. Use only for homogeneous lesions. Caveats: ~⅓ of pheochromocytomas wash out in the adenoma range, ~⅓ of adenomas do not meet criteria, and some malignancies (ACC, hypervascular mets) can mimic adenoma washout.

Pheochromocytoma features

Non-contrast attenuation is almost always >10 HU. Portal-venous hyperenhancement (pheochromocytoma mean ~93 HU) is a soft, nonspecific sign with no validated cutoff. Marked T2 hyperintensity is the stronger discriminator: T2 SI ratio >3.8 (AUC 0.91; mean pheo 4.4 vs adenoma 1.8). Plasma free metanephrines are mandatory for any indeterminate mass >10 HU.

Chemical-shift MRI

SII = (SI in-phase − SI opposed-phase)/SI in-phase × 100; adenoma if >16.5% (1.5 T) or >7.4% (3 T). ASR = (adrenal OP/spleen OP)/(adrenal IP/spleen IP); adenoma if <0.71. Less reliable when CT attenuation >30 HU. Clear-cell RCC and HCC metastases may contain fat and mimic adenoma.

Size

Size-specific ACC risk (Kahramangil, Ann Surg 2022; 2,219 incidentalomas): <4 cm 0.1%, 4–6 cm 2.4%, >6 cm 19.5% (ROC-optimal cutoff 4.6 cm). ≥4 cm → substantially higher risk → MRI or PET/CT; ≥6 cm → surgical threshold at many centers. Size must never be used in isolation — integrate with HU density, hormonal evaluation, and growth kinetics (AAES 2022).

Growth

Corwin (AJR 2019): all 105 adenomas that grew did so <3 mm/yr and all 26 malignant nodules grew >5 mm/yr, so a 3 mm/yr cutoff gave 100% sensitivity/specificity — the discriminating cutoff is 3 mm/yr, not 5. Caveats: retrospective, only 26 malignant nodules (including fast-growing metastases), wide CI (sensitivity 86.8–100%). A larger multicenter series of atypical masses (Balderrama-Brondani, JCEM 2025) found a best cutoff of ~2.68 mm/yr (AUC 0.939, sens 87.5%, spec 88.8%), so Corwin likely overstates discrimination. CUA/AUA uses >5 mm/yr as a surgical threshold (weak recommendation, low-quality evidence). ESE/ENSAT: consider resection if maximum diameter increases >20% AND ≥5 mm. Stability over 6–12 months usually favors benignity, but rare ACCs have arisen after years of stability in radiologically indeterminate lesions — stability is not absolute reassurance (Howarth, JCEM 2026).

FDG-PET/CT

Tumor:liver SUVmax ≤1.0 favors benign; 1.0–1.5 grey zone; >1.5 suggestive of malignancy; >5 highly suspicious.

Dual-energy CT

Fat fraction ≥23.8% (spec 100%); iodine-to-VNC ratio = iodine ÷ VNC, ≥6.7 (sens/spec ~95%); relative enhancement ratio = portal venous ÷ VNC × 100, ≥214% (spec 100%). VNC overestimates true unenhanced HU (~+11 HU), so the ≤10 HU rule is unreliable on VNC.

Primary sources: CUA/AUA guideline (Rowe, J Urol 2023); AAES guideline (Yip, JAMA Surg 2022); Kahramangil, Ann Surg 2022 (size-specific ACC risk); Corwin, AJR 2026 (malignancy prevalence in incidental indeterminate nodules); Corwin, AJR 2019 & Balderrama-Brondani, JCEM 2025 (growth); Howarth, JCEM 2026 (ACC after stability); Kebebew, NEJM 2021; ACR Appropriateness Criteria 2021; Nagayama Radiology 2020; Loonis AJR 2023; Ream AJR 2015. Most growth/size recommendations are weak, low-quality evidence — interpret with clinical and biochemical context.

Characterizing adrenal nodules

How each imaging method separates benign adenoma/myelolipoma from pheochromocytoma, adrenocortical carcinoma, and metastasis. Expand any method for principle, thresholds, performance, and pitfalls. Any indeterminate mass >10 HU needs biochemical screening (plasma free metanephrines) before biopsy.

Method	Type	Sens	Spec	Key threshold	Favors	Lipid-poor adenoma?
Non-contrast CT	Quant	71%	98%	≤10 HU = adenoma	benign	No
CT washout (APW)	Quant	~78%	~70%	APW ≥60% / RPW ≥40%	benign	Yes (if homogeneous)
Chemical-shift MRI	Quant	~89–93%	~90%	SII >16.5% (1.5T) / >7.4% (3T); ASR <0.71	benign	No
Size	Quant	mod	57% (>4 cm)	≥4 cm risk; ≥6 cm surgical	malignant	n/a
Growth	Quant	100%	100%	<3 mm/yr = adenoma (malignancies all >5 mm/yr; Corwin 2019)	benign	n/a
Dual-energy CT	Quant	~85%	~88%	FF ≥23.8%; iodine:VNC ≥6.7; RER ≥214%	benign	partial
FDG-PET/CT	Quant	87%	85%	tumor:liver SUVmax >1.5	malignant	n/a

Sens/Spec refer to the call named in Favors: for benign-favoring rows they are for diagnosing adenoma; for malignant-favoring rows (Size, FDG-PET) they are for detecting malignancy.

1 · Non-contrast CT attenuationFirst-line

Principle

Adenomas are rich in intracytoplasmic lipid, which lowers attenuation. Malignancy and pheochromocytoma lack it. Macroscopic fat (well below 0 HU) indicates myelolipoma.

Thresholds & performance

≤10 HU = lipid-rich adenoma: sensitivity 71%, specificity 98% (near-100% specific).
10–20 HU indeterminate (up to 7% of metastases fall here); >30 HU substantially raises malignancy likelihood.
Pheochromocytoma is almost always >10 HU (374/376 in the largest series).

Pitfalls

~30% of adenomas are lipid-poor (>10 HU) and the ≤10 HU rule misses them all.

Boland et al. AJR 1998; ACR Appropriateness Criteria 2021; Kebebew NEJM 2021.

2 · CT washout (APW / RPW)

Principle

Adenomas (lipid-rich and lipid-poor) de-enhance faster than most malignancy. Requires unenhanced + portal-venous (60–75 s) + delayed (10–15 min) phases.

Thresholds

APW ≥60% = (enhanced − delayed)/(enhanced − unenhanced) × 100.
RPW ≥40% = (enhanced − delayed)/enhanced × 100.
Use only for homogeneous lesions.

Caveats (important)

~1/3 of pheochromocytomas wash out in the adenoma range; ~1/3 of adenomas do not meet criteria.
Hypervascular metastases (clear-cell RCC, HCC) can mimic adenoma washout.
ESE 2023 did not recommend washout CT; AAES 2022 gave only a weak recommendation; large studies (Corwin 2022, Schloetelburg 2022) found thresholds insufficient to reliably characterize lesions.

Caoili Radiology 1998; Korobkin AJR 1998; Corwin AJR 2022; Schloetelburg Eur J Endocrinol 2022; Yip JAMA Surg 2022 (AAES).

3 · Washout pitfalls — pheo / ACC overlapDon't use alone

Key data

Up to 30–35% of pheochromocytomas show APW ≥60% (adenoma range); mean APW 31.9% ± 42.8% with extreme variability.
Pheochromocytomas show significantly higher portal-venous enhancement (mean ~93 HU) — hyperenhancement is the better discriminator even when washout overlaps.
ACC and hypervascular metastases can also overlap.

Implication

Washout CT must never be used in isolation to exclude pheochromocytoma. Plasma free metanephrines are mandatory for any mass >10 HU; morphology (size >6 cm, necrosis, heterogeneity) should override washout values.

Schieda AJR 2016; Blake RadioGraphics 2004 ("Imaging Chameleon"); Pacak Lancet Diabetes Endocrinol 2026.

4 · Chemical-shift MRI

Principle

Opposed-phase signal loss reflects intracellular lipid (adenoma). Lesions without lipid (metastasis, pheo, ACC) do not drop.

Thresholds & performance

SII = (SI in-phase − SI opposed-phase)/SI in-phase × 100. Adenoma if >16.5–21.6% at 1.5 T, >7.4% at 3 T.
ASR <0.71 = adenoma.
Sensitivity ~89–93%, specificity ~90% at both field strengths.

Pitfalls

Lipid-poor adenomas may not drop — same fundamental limit as non-contrast CT.
Only homogeneous drop is diagnostic; minute lipid occurs in pheo/ACC/some metastases.
The 1.5 T threshold produces false negatives at 3 T — use the field-specific cutoff.

Platzek Eur Radiol 2019 (meta-analysis); Ream AJR 2015 (1.5T vs 3T); Schieda AJR 2016.

5 · Size & growth

Size thresholds

<4 cm low risk; 4–6 cm intermediate; >6 cm high risk / surgical at many centers.
Size-specific ACC risk (Kahramangil, Ann Surg 2022; n=2,219): <4 cm 0.1%, 4–6 cm 2.4%, >6 cm 19.5%; ROC-optimal cutoff 4.6 cm.
Size >4 cm alone: specificity only ~57% — most large adrenal masses are still benign adenomas. Never use size in isolation (AAES 2022).
Incidental indeterminate nodules on contrast-enhanced CT, no cancer history (Corwin, AJR 2026; 12 institutions, 1,506 nodules): malignancy 0/773 at 1–2 cm (95% CI 0–0.4%), 1/694 at 2–4 cm, 0/39 over 4 cm; authors conclude 1–2 cm nodules need no follow-up. Of 129 nodules over 20 HU on later unenhanced CT, none was malignant, which argues against ESE 2023 treating >20 HU as high risk on its own. Limits: retrospective, mostly imaging or clinical reference standard, no biochemistry (pheochromocytoma 0.3%, likely undercounted).

Growth

3 mm/year is the statistically derived cutoff: in Corwin (AJR 2019) all adenomas that grew did so <3 mm/yr and all malignant nodules grew >5 mm/yr, giving 100% sens/spec at 3 mm/yr (not 5 mm/yr).
Corwin caveats: retrospective, only 26 malignant nodules (including fast-growing metastases), wide CI (sens 86.8–100%). Newer multicenter data (Balderrama-Brondani, JCEM 2025) give a best cutoff ~2.68 mm/yr (AUC 0.939; sens 87.5% / spec 88.8%) — Corwin likely overstates performance.
>5 mm/year: CUA/AUA surgical threshold (weak recommendation, low-quality evidence).
ESE/ENSAT: consider resection if >20% increase in maximum diameter and ≥5 mm absolute growth.
Stability over 6–12 months usually favors benignity, but rare ACCs have arisen after years of stability in indeterminate lesions — not absolute reassurance (Howarth, JCEM 2026).

Kahramangil Ann Surg 2022; Corwin AJR 2019; Corwin AJR 2026 (prevalence); Balderrama-Brondani JCEM 2025; Rowe (CUA/AUA) J Urol 2023; Yip (AAES) JAMA Surg 2022; ESE/ENSAT guideline; Howarth JCEM 2026.

6 · Dual-energy CTSingle-phase capable

Principle

Material decomposition yields fat fraction, iodine quantification, and virtual non-contrast (VNC) — can characterize fat even from a contrast-enhanced acquisition.

Thresholds

Fat fraction ≥23.8% (spec ~100%).
Iodine:VNC ratio ≥6.7 (sens/spec ~95%).
Relative enhancement ratio (RER) = portal-venous HU ÷ VNC × 100, ≥214% (spec ~100%).
VNC overestimates true unenhanced HU (~+11 HU), so the ≤10 HU rule is unreliable on VNC.

Loonis AJR 2023; Nagayama Radiology 2020; Mileto Radiology 2015; Bernard Abdom Radiol 2025.

7 · FDG-PET/CT

Thresholds & performance

Adrenal-to-liver SUVmax ratio >1.45–1.5: sensitivity 87–90%, specificity 86–93% for malignancy.
1.0–1.5 grey zone (usually benign); >5 seen in ACC, lymphoma, metastasis, pheo.
Pooled meta-analysis: sensitivity 87.3%, specificity 84.7% (Schaafsma 2023).
Non-cancer patients: liver ratio >2.5 (sens 85% / spec 90%, He 2021).

Pitfalls

Pheochromocytomas are FDG-avid (false positives); low-grade ACC and RCC metastases can be FDG-low (false negatives); inflammation/brown fat confound.

Groussin JCEM 2009; Guerin JCEM 2017; Schaafsma JCEM 2023; He JCEM 2021.

8 · Pheochromocytoma featuresBiochemistry mandatory

Features

Non-contrast almost always >10 HU (rarely exactly 10, never <10 in the largest series).
Marked T2 hyperintensity: T2 SI ratio >3.8 specific (AUC 0.91); mean pheo 4.4 ± 2.4 vs adenoma 1.8 ± 0.8.
Portal-venous hyperenhancement (mean ~93 HU) — soft, nonspecific (no validated cutoff).
No CSI signal drop (mean SII −3.5%); rare lipid-containing pheo is a pitfall. Often heterogeneous, cystic/necrotic, hemorrhagic.

Reliability

No single feature is diagnostic; plasma free metanephrines are mandatory for any indeterminate mass >10 HU.

Blake RadioGraphics 2004; Raja AJR 2013; Schieda AJR 2016; Neumann NEJM 2019.

9 · ACC & metastasis features

Adrenocortical carcinoma

Usually >6 cm; >10 HU on non-contrast; heterogeneous with irregular margins.
Necrosis is near-universal — CT-detected necrosis had ~98% accuracy for ACC (OR 35 vs adenoma).
Delayed washout (APW <60%), avid/heterogeneous FDG uptake, venous extension (highly specific), calcification.

Metastasis

Variable size; >10 HU (up to 7% measure 10–20 HU); heterogeneous, irregular margins; can be bilateral.
Usually slow washout, but hypervascular mets (RCC, HCC) can mimic adenoma washout; T2-hyperintense, no CSI drop; growth >5 mm/yr.

Kebebew NEJM 2021; Garay-Lechuga Surgery 2022 (necrosis); Corwin AJR 2019 (growth).

Guideline library

CUA/AUA incidental adrenal mass guideline — Rowe NE, et al. J Urol 2023;210(4):590–599.
Adrenal Incidentaloma review — Kebebew E. NEJM 2021;384(16):1542–1551.
ACR Appropriateness Criteria: Adrenal Mass Evaluation (2021 update) — JACR 2021;18(11S):S251–S267.
AAES adrenalectomy guidelines — Yip L, et al. JAMA Surg 2022;157(10):870–877.
Chemical-shift MRI meta-analysis — Platzek I, et al. Eur Radiol 2019;29(2):806–817.
FDG-PET meta-analysis — Schaafsma M, et al. JCEM 2023;108(9):2435–2445.
Adrenal growth rates — Corwin MT, et al. AJR 2019;213(3):632–636.
Adrenal tumour imaging review — Pacak K, et al. Lancet Diabetes Endocrinol 2026;14(1):62–81.

Educational summary compiled from the above; numbers reflect the cited primary literature and vary by population and technique.