Adrenal Lesions
- Adrenal lesions are mostly incidental, mostly benign, and mostly adenomas — the goal of imaging is to confidently say "this is nothing" without a needle.
- The single most useful trick on CT is fat: an adenoma packed with fat measures low (the classic threshold is ≤10 Hounsfield units on a non-contrast scan).
- If a lesion isn't fatty enough to call on the unenhanced scan, washout on a dedicated adrenal CT protocol is the next move — adenomas wash contrast out fast.
- Size and growth matter: a small, stable, fatty nodule is reassuring, while a large, growing, or heterogeneous mass earns more worry.
- Two things make a "lesion" actually dangerous: it secretes hormones, or it's malignant (often a metastasis in a cancer patient). Imaging plus biochemistry sorts this out together.
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. 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.
The fat trick: why density is everything
Here's the elegant part. The most common adrenal lesion by far is the adenoma, a benign overgrowth, 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; it barely slows the beam down. So on a non-contrast CT, that fatty lesion measures low.
The radiologists put a number on this: a lesion that measures ≤10 Hounsfield units (HU) on an unenhanced CT is confidently called a benign lipid-rich adenoma. (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.
A homogeneous adrenal nodule measuring ≤10 HU on non-contrast CT is a benign adenoma. That single number resolves most adrenal incidentalomas.
When fat doesn't bail you out: 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 that's where a dedicated adrenal CT protocol earns its keep.
The idea is timing. You scan before contrast, right after contrast, and then again after a 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. So you watch how fast the enhancement fades: brisk washout points to a benign adenoma, sluggish washout keeps the lesion on the suspect list. It's the difference between a paper towel that releases water the moment you lift it and a brick that stays soaked.
The rogues' gallery
Adenomas dominate, but the adrenal hosts a whole cast of characters. A quick differential:
| Lesion | Typical clue | Worry level |
|---|---|---|
| Adenoma | Low density (≤10 HU) and/or fast washout; usually small and stable | Low — by far the most common |
| Myelolipoma | Contains macroscopic fat (clearly fat-density, like the fat elsewhere in the belly) | Low — benign, just don't mistake it for anything sinister |
| Metastasis | Known primary cancer, often bilateral, dense, may grow on follow-up | High — the main reason to take a new adrenal nodule seriously in a cancer patient |
| Pheochromocytoma | Often vivid enhancement, can be large/heterogeneous; secretes catecholamines | High — hormonally dangerous |
| Adrenocortical carcinoma | Large, heterogeneous, irregular, often growing | High — rare but aggressive |
Macroscopic fat (a myelolipoma) and microscopic intracellular fat (a lipid-rich adenoma) are not the same thing, even though both involve "fat." Macroscopic fat is chunky, obvious, fat-density tissue you can see; intracellular fat is microscopic and only shows up as a low average HU measurement. Both are benign — but they're different diagnoses, so name them precisely.
Size, stability, and the secreting troublemakers
Beyond density, two features nudge the worry meter. Size matters: small lesions are overwhelmingly benign, while a larger adrenal mass — especially one that's heterogeneous or irregular — raises suspicion for adrenocortical carcinoma. And growth over time is a red flag; a nodule that's been identical on scans for years is reassuring, while one that's clearly enlarging needs attention.
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.
Don't reach for a biopsy needle on an adrenal mass until a pheochromocytoma has been excluded biochemically. Sticking a needle into an unsuspected pheo can trigger a dangerous catecholamine surge. When in doubt, the lab goes before the needle.
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-washout 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 non-contrast HU measurement settles the question more often than not. When it doesn't, washout and a phone call to endocrinology usually do the rest.