Adrenal Lesions

You weren't even looking at the adrenal glands. You ordered the CT for belly pain, or to stage a known cancer, and there it is — a little blob sitting on top of the kidney like a beret on a snowman. Welcome to the adrenal incidentaloma, one of the most common "oops, found something" moments in all of imaging. The good news: the overwhelming majority of these are harmless. The job is proving it efficiently, without sending everyone to the surgeon.

What the glands actually are

Each adrenal gland is a small, Y- or V-shaped structure perched above its kidney. Picture a flattened pat of butter draped over the top of each kidney — soft, thin-limbed, and easy to overlook. The outer cortex makes steroid hormones (cortisol, aldosterone, androgens); the inner medulla makes catecholamines (the adrenaline family). When something rounds out one of those limbs into a discrete nodule, that's your lesion.

The two questions every adrenal lesion has to answer are simple, even if the workup isn't: Is it making hormones it shouldn't? and Is it cancer? Imaging is brilliant at the second question and useless at the first — that one belongs to the lab. So a good radiologist always thinks of the adrenal as a team sport with endocrinology.

Findings by modality

CT, unenhanced

The most common adrenal lesion by far is the adenoma, a benign overgrowth of the cortex, and a lot of adenomas are stuffed with intracellular fat — think of a tiny sponge soaked in oil instead of water. Fat is light to an X-ray beam, so on an unenhanced CT that fatty lesion measures low.

The radiologists put a number on this: a homogeneous lesion that measures ≤10 Hounsfield units (HU) on an unenhanced CT is a benign lipid-rich adenoma, regardless of its size. (Hounsfield units are just the CT density scale — water sits at zero, fat goes negative, soft tissue runs higher.) That one measurement closes the case for a huge fraction of incidentalomas. No follow-up, no biopsy, no drama.

The same series hands you two more free passes. A mass containing macroscopic fat — chunky, obvious, fat-density tissue like the fat elsewhere in the belly — is a myelolipoma, benign, no follow-up. And a mass that doesn't enhance at all, changing by less than 10 HU between the unenhanced and enhanced images, is a cyst or a hematoma, also done.

Figure · CT

CT, adrenal protocol (washout)

Not every adenoma is generously oily. Some are lipid-poor and measure above 10 HU, so the density trick fails. These are indeterminate on the plain scan — and for an indeterminate mass 1 to <4 cm, the ACR white paper names a dedicated adrenal-protocol CT as the examination of choice.

The idea is timing. You scan before contrast (reviewed in real time, so the study stops early if the mass is already ≤10 HU), scan again on a dynamic phase at 60–90 seconds after a power injection, and then again at a 15-minute delay. Adenomas, oily or not, grab contrast and then let it drain away quickly — they have fast washout. Most other lesions, including malignancies, hold onto contrast stubbornly. It's the difference between a paper towel that releases water the moment you lift it and a brick that stays soaked. The full formulas and a worked example live on the adenoma-versus-metastasis washout page; the cutoffs are absolute washout ≥60% and relative washout ≥40%.

MRI

MRI's version of the fat trick is chemical-shift imaging: in-phase and opposed-phase T1 images. A lipid-rich adenoma loses signal on the opposed-phase images compared with in-phase, judged against the spleen; the ACR considers that drop almost always diagnostic of an adenoma regardless of size, which makes MRI a fine way to characterize an adenoma when CT is indeterminate or the patient shouldn't have iodinated contrast. Pheochromocytoma is classically bright on T2 and enhances avidly; adrenocortical carcinoma is large and heterogeneous; myelolipoma follows fat on every sequence.

Nuclear medicine

Functional imaging answers questions CT can't. MIBG scintigraphy targets catecholamine-handling tissue and hunts for pheochromocytoma and paraganglioma; FDG PET/CT is the problem-solver in a cancer patient with an indeterminate mass, and the ACR floats it (or biopsy) when a mass shows central necrosis, which raises the likelihood of metastasis.

The numbers

How good is the test

The unenhanced-CT threshold trades sensitivity against specificity, and the pooled literature shows the trade explicitly: across 495 adrenal lesions from ten CT reports (272 benign, 223 malignant), sensitivity for benign disease rose from 47% at 2 HU to 88% at 20 HU, while specificity fell from 100% at 2 HU to 84% at 20 HU. 10 HU is the conventional compromise — highly specific, so a "benign" call is trustworthy, at the cost of leaving lipid-poor adenomas undecided.

Adding the washout step closes most of that gap: the combined criteria (≤10 HU unenhanced or washout ≥60%) correctly categorized nearly all of 166 adrenal masses (127 adenomas, 39 non-adenomas) in the study that established the protocol. For chemical-shift MRI and for PET/CT, the concept is well established but there is no robust numeric accuracy figure to quote, so none is stated here.

The ACR pathway at a glance

Abbreviated from the ACR Incidental Findings Committee white paper (Mayo-Smith et al., 2017). The full algorithm, including the branch for patients with a known cancer, lives in the paper.

The rogues' gallery

Adenomas dominate, but the adrenal hosts a whole cast of characters.

Size, stability, and the secreting troublemakers

Beyond density, two features nudge the worry meter. Size matters: the ACR draws its line at 4 cm — an isolated mass at or above it, with no benign diagnostic features and no cancer history, is resected without biopsy to treat possible adrenocortical carcinoma. And growth over time is a red flag, but not a formula: the white paper says plainly that no growth rate separates benign from malignant, so an enlarging indeterminate mass earns biochemical evaluation and a specialist conversation rather than a threshold. A nodule that's been identical on scans for years, on the other hand, is reassuring all by itself.

The sneaky ones are the functional lesions — the pheochromocytoma chief among them. It can look like a banal mass but be flooding the body with catecholamines, causing spells of pounding headaches, sweating, and blood pressure that swings like a faulty thermostat. Imaging can hint at it (often striking enhancement), but you can't exclude it from pictures alone — that's a biochemistry diagnosis. The three non-adenoma characters get their own page: myelolipoma, pheochromocytoma & carcinoma.

Reporting

• Size (largest dimension, flagged against the 1 to <4 cm and ≥4 cm bands) and side.
• Unenhanced attenuation in HU and whether the mass is homogeneous; if ≤10 HU, say "lipid-rich adenoma; no further imaging."
• Diagnostic benign features named explicitly: macroscopic fat, no enhancement.
• Washout values when a protocol study was done — the three HU numbers and the calculated APW or RPW against its cutoff.
• Worrying features: heterogeneity, central necrosis, invasion, venous tumor thrombus, bilaterality in a cancer patient.
• Comparison with priors and the stability interval.
• The recommendation: nothing; adrenal-protocol CT or chemical-shift MRI; biochemical evaluation; or surgical referral.

How this fits the bigger picture

The adrenal incidentaloma is a perfect example of the broader art of managing incidental findings — knowing when a small dense blob deserves a full workup and when it deserves a shrug. The same density-and-enhancement logic that tames the adrenal also rhymes with how we characterize renal masses next door, and the hunt for adrenal metastases is part of why we scrutinize the glands in any patient with a known cancer.

If you remember one thing: most adrenal lesions are benign adenomas, and a single unenhanced HU measurement — ≤10 — settles the question more often than not. When it doesn't, washout (≥60% absolute, ≥40% relative) and a phone call to endocrinology usually do the rest.

References

• Mayo-Smith WW, Song JH, Boland GL, et al. Management of Incidental Adrenal Masses: A White Paper of the ACR Incidental Findings Committee. J Am Coll Radiol 2017;14(8):1038–1044. Used for: the ≤10 HU unenhanced threshold, the ≥60% absolute and ≥40% relative washout cutoffs and formulas, the <10 HU non-enhancing definition, macroscopic fat and the benign calcified mass as diagnostic benign features, the adrenal-protocol timing (unenhanced, 60–90 s dynamic, 15-min delay), the 1 to <4 cm and ≥4 cm size rules, the "no growth-rate threshold" statement, the central-necrosis guidance, and the chemical-shift MRI criterion in "Findings by modality," "The numbers," "The ACR pathway at a glance," "The rogues' gallery," "Size, stability, and the secreting troublemakers," and "Reporting."
• Caoili EM, Korobkin M, Francis IR, et al. Adrenal masses: characterization with combined unenhanced and delayed enhanced CT. Radiology 2002;222(3):629–633. Used for: the ≤10 HU and ≥60% washout criteria and the 166-mass (127 adenomas, 39 non-adenomas) result in "How good is the test."
• Boland GW, Lee MJ, Gazelle GS, Halpern EF, McNicholas MM, Mueller PR. Characterization of adrenal masses using unenhanced CT: an analysis of the CT literature. AJR Am J Roentgenol 1998;171(1):201–204. Used for: the sensitivity (47% at 2 HU to 88% at 20 HU) and specificity (100% at 2 HU to 84% at 20 HU) ranges, the 495-lesion (272 benign, 223 malignant; ten reports) pooled analysis, and the 10 HU compromise in "How good is the test."
• Israel GM, Korobkin M, Wang C, Hecht EN, Krinsky GA. Comparison of unenhanced CT and chemical shift MRI in evaluating lipid-rich adrenal adenomas. AJR Am J Roentgenol 2004;183(1):215–219. Used for: the chemical-shift (opposed-phase signal loss) concept in "Findings by modality"; no numeric index is taken from it.