Adrenal Myelolipoma, Pheochromocytoma & Carcinoma
- These are the three adrenal masses that aren't the usual boring adenoma — and each gives itself away differently.
- Myelolipoma = macroscopic fat. Find bulk fat in an adrenal mass and you can basically relax; it's benign.
- Pheochromocytoma = a hormone factory that loves to look bright and lush on T2 MRI and enhance avidly. The catch is what it does to blood pressure.
- Adrenocortical carcinoma (ACC) = the big, ugly, heterogeneous one that invades and metastasizes. Size and growth are red flags.
- Macroscopic fat (myelolipoma) is different from the microscopic fat that defines a benign adenoma — don't confuse the two.
Most adrenal lumps are dull little benign adenomas, and the whole game there is the adenoma-versus-metastasis washout workup. This page is about the three that break that mold — the fatty one, the explosive one, and the scary one. Think of it as the adrenal gland's character cast: the harmless hoarder, the chaotic gremlin, and the villain.
The harmless hoarder: myelolipoma
A myelolipoma is what you'd get if a chunk of bone marrow wandered into the adrenal gland and decided to stay. It's a benign mix of mature fat and scattered blood-forming (myeloid) elements — basically a fat blob with some marrow sprinkled in.
The good news: you can usually spot it cold. The defining feature is macroscopic fat — big, honest, dietary-style fat, the same density as the fat under your skin.
On CT, look for chunks of tissue measuring around the density of subcutaneous fat — distinctly negative Hounsfield units. Bulk fat inside an adrenal mass is the signature of a myelolipoma, and it's reassuringly benign.
This is where people trip, so let me draw the line clearly. A benign adenoma also contains fat, but it's microscopic — fat sprinkled inside individual cells, like marbling in a steak, which is why adenomas drop their density and lose signal on chemical-shift MRI. A myelolipoma has macroscopic fat — visible pockets and globs, like the fat trimmings you'd cut off the steak. Same word, totally different scale, totally different lesion.
Don't mistake macroscopic fat for the microscopic fat of an adenoma. Macroscopic fat = visible globs at fat density on CT (myelolipoma). Microscopic fat = intracellular, invisible as discrete fat but detectable by signal dropout on opposed-phase chemical-shift MRI (adenoma). One is marbling; one is trimmings.
Hounsfield units are the language we're speaking here, so if "negative HU equals fat" feels shaky, take two minutes on Hounsfield units and windowing.
The chaotic gremlin: pheochromocytoma
A pheochromocytoma is a tumor of the adrenal medulla — the inner core that makes adrenaline-type hormones (catecholamines). When it misbehaves, it dumps those hormones into the bloodstream in bursts, and the classic clinical picture is episodic headaches, sweating, palpitations, and spiky high blood pressure. It's the tumor that turns a patient into a fire alarm that goes off at random.
On imaging it tends to be vivid:
- CT: a soft-tissue mass that often enhances avidly with contrast and can show cystic or hemorrhagic change as it outgrows its blood supply.
- MRI: classically bright on T2 — older teaching called this the "light bulb" appearance, though plenty of pheos are more variable than the textbooks promise, so don't bank everything on it.
There's an old worry that injecting iodinated contrast could trigger a catecholamine surge ("pheo crisis"). With modern low-osmolar contrast this risk is now considered low, but the tumor's reputation for drama is why it's flagged everywhere. Know your local protocol and the clinical context.
Pheochromocytoma is also a card-carrying member of the "10% tumor" lore — roughly 10% extra-adrenal, 10% bilateral, 10% malignant, and so on. Treat these as memory pegs, not exact statistics; the real numbers shift with genetics and series, and a meaningful chunk of cases are tied to hereditary syndromes.
When a pheo lives outside the adrenal (a paraganglioma) or you need to hunt for hidden ones, functional imaging steps in — see the adrenal MIBG scan, which targets catecholamine-handling tissue specifically.
The villain: adrenocortical carcinoma
Adrenocortical carcinoma (ACC) is the rare, aggressive cancer of the adrenal cortex. If myelolipoma is the harmless hoarder and pheo is the gremlin, ACC is the one wearing a black hat.
The imaging vibe is big, ugly, and heterogeneous. These tumors are often large at diagnosis, with messy internal enhancement, areas of necrosis, hemorrhage, and sometimes calcification — the look of a tumor that has outgrown its own plumbing.
The two findings that should make your stomach drop are invasion and spread: ACC likes to push into the adjacent kidney, grow as tumor thrombus up the adrenal/renal vein into the inferior vena cava, and metastasize (lungs and liver are common targets).
| Feature | Reassuring | Worrisome (think ACC) |
|---|---|---|
| Size | Small, stable | Large and/or growing on follow-up |
| Margins | Smooth, well-defined | Irregular, infiltrative |
| Internal texture | Homogeneous | Heterogeneous: necrosis, hemorrhage, calcification |
| Local behavior | Stays put | Invades kidney, vein thrombus, distant mets |
Size and growth matter. There's no single magic threshold that proves cancer, but a large adrenal mass — and especially one that enlarges over time — earns a hard look and usually a specialist conversation rather than a casual "probably nothing."
The one-breath summary
Three masses, three tells. Fat in a glob? Myelolipoma — exhale. T2-bright, avidly enhancing, and the patient's blood pressure is doing interpretive dance? Pheochromocytoma — think hormones and handle with care. Big, heterogeneous, invading the neighbors? Adrenocortical carcinoma — sound the alarm. Match the picture to the character and the adrenal gland stops being mysterious.