Adrenal Adenoma vs Met (washout)
- Adenomas are the friendly couch potatoes of the adrenal world: super common, almost always benign, and frequently stuffed with fat.
- A lipid-rich adenoma gives itself away on non-contrast CT by being low density — usually ≤10 Hounsfield units — because all that internal fat drags the number down.
- Lipid-poor adenomas don't have the fat tell, so we lean on washout: adenomas grab contrast and then let go of it fast, while metastases hold onto it stubbornly.
- The two numbers to know are absolute washout (≥60%) and relative washout (≥40%) — high washout points to adenoma.
- Context matters enormously: an adrenal nodule in someone with no cancer is almost always nothing; in someone with a known malignancy, you owe it a real look.
You scan a belly for a kidney stone and there it is — a little lump sitting on top of the kidney like a hat. An adrenal nodule. Half the time it means absolutely nothing, and half the time it's the one finding that changes a cancer patient's whole staging. The job is telling those two apart without poking the patient with a needle. Good news: the adrenal gland practically hands you the answer if you know which questions to ask.
The cast of characters
The adrenal adenoma is the overwhelming favorite. These show up in a meaningful chunk of routine abdominal CTs as an incidentaloma — a thing you weren't looking for that decided to come to the party anyway (see incidentaloma frameworks). Most are non-functioning, meaning they sit there quietly and don't pump out extra hormones. The plot twist that makes them easy to catch: a lot of them are packed with intracellular fat.
The villain we're ruling out is the metastasis. Adrenals are a popular landing spot for spread from lung, breast, melanoma, kidney, and others — partly because they have a rich blood supply, like a busy highway off-ramp. A met has no reason to be full of fat, and it tends to be greedy with contrast.
"Adenoma vs met" is the everyday question, but it isn't the whole zoo. Pheochromocytoma, myelolipoma, and adrenal cortical carcinoma all live here too and behave differently — those get their own treatment in adrenal myelolipoma, pheochromocytoma & carcinoma.
The fat tell: non-contrast density
Here's the elegant part. Fat is less dense than water and soft tissue, so it reads low on CT. If an adrenal nodule is crammed with intracellular fat, its average Hounsfield unit value — the radiologist's density scale, with water set at zero — gets pulled down.
The rule of thumb: on a non-contrast CT, a homogeneous nodule measuring ≤10 HU is a lipid-rich adenoma, and you can essentially call it benign and walk away. That's it. One measurement, one circle, done. It feels almost too easy, and most of the time it is.
When the fat trick fails: washout
Roughly a third of adenomas are lipid-poor — they skipped the fat and measure above 10 HU on non-contrast CT, sitting right in the gray zone with the mets. Annoying. This is where the adrenal washout CT earns its keep.
The concept is about timing, not just brightness. You give IV contrast, scan in the early (portal venous, ~60–75 second) phase, then scan again on a delayed phase (commonly ~15 minutes). Both adenomas and mets light up early. The difference is what happens next: an adenoma releases contrast quickly — it washes out — while a metastasis clings to it and stays bright. Think of a paper towel versus a sponge that's already saturated: pour water on both, and the paper towel sheds it fast while the sponge just sits there heavy and wet.
We turn that into two numbers, using the early enhanced density (E), the delayed density (D), and — for absolute washout — the non-contrast density (U):
| Calculation | Formula | Favors adenoma |
|---|---|---|
| Absolute washout (needs non-contrast) | (E − D) ÷ (E − U) × 100 | ≥ 60% |
| Relative washout (no non-contrast needed) | (E − D) ÷ E × 100 | ≥ 40% |
High washout = adenoma. The adrenal that grabs contrast and then dumps it fast is behaving like an adenoma; the one that holds on is behaving like a met.
The traps worth naming
The ≤10 HU rule is a non-contrast rule. Measuring 10 HU on a contrast-enhanced scan tells you nothing useful — enhancement raises the number, so a perfectly benign adenoma can read 40 or 50 HU after contrast and look scary. If all you have is a portal venous scan, you can't apply the threshold; you need the dedicated non-contrast or washout protocol.
Washout has its own fine print. The classic thresholds were validated for typical adenomas, and a few other lesions can wash out convincingly — pheochromocytoma being the famous impostor — so the numbers are a strong clue, not a magic wand. And context is king: a small, stable, homogeneous nodule in a healthy person almost never needs a workup, while the same nodule in someone with known lung cancer deserves the full investigation, and sometimes a biopsy or an MIBG-type problem-solving study. Stability over prior imaging is its own kind of evidence — a nodule that's looked identical for years is reassuring all on its own.
Before ordering a fancy washout protocol, go fishing for old scans. A nodule that's been sitting there unchanged for a couple of years has already passed the most reliable test there is: time.
The bottom line
Most adrenal nodules are sleepy adenomas. Reach for non-contrast density first — ≤10 HU and you're done. If it's lipid-poor and stuck in the gray zone, let time break the tie with a washout study, remembering that adenomas shed contrast and mets hang on. Wrap every measurement in the patient's story, and that little hat on the kidney usually turns out to be exactly as boring as it looks.