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 relax: the ACR calls it a diagnostic benign feature needing no further workup or follow-up.
- 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 — and it's a biochemical diagnosis, not an imaging one.
- Adrenocortical carcinoma (ACC) = the big, ugly, heterogeneous one. An isolated adrenal mass ≥4 cm with no benign features and no cancer history goes to surgical resection without biopsy.
- 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.
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 average 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 as a low average attenuation (≤10 HU) or as signal dropout on opposed-phase chemical-shift MRI (adenoma). One is marbling; one is trimmings.
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.
The single most important thing to know is that this is a biochemical diagnosis: plasma or urine metanephrines. Imaging tells you where it is and what it looks like; it cannot tell you what it's secreting, and it cannot exclude a pheo. A meaningful share of cases travel with hereditary syndromes, and a pheo that lives outside the adrenal is called a paraganglioma.
There's an old worry that injecting iodinated contrast could trigger a catecholamine surge ("pheo crisis"). That concern has receded with modern low-osmolar agents, but the tumor's reputation for drama is why it's flagged everywhere — and it's the reason you never put a biopsy needle into an adrenal mass until a pheo has been excluded biochemically. The lab goes before the needle.
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 or renal vein into the inferior vena cava, and metastasize (lungs and liver are common targets).
Findings by modality
CT
- Myelolipoma: a well-circumscribed mass containing regions of frank fat attenuation — negative Hounsfield units matching the subcutaneous or retroperitoneal fat — interspersed with soft-tissue strands (the marrow). Sometimes the fat is nearly the whole lesion; sometimes it's a few pockets. Calcification and hemorrhage can occur. If "negative HU equals fat" feels shaky, take two minutes on Hounsfield units and windowing.
- Pheochromocytoma: a soft-tissue mass that fails the adenoma tests — it isn't fatty, so it doesn't measure ≤10 HU unenhanced — and typically enhances avidly on the dynamic phase, with cystic or hemorrhagic change as it outgrows its blood supply. Beware: a pheo can occasionally wash out on the delayed phase like an adenoma, which is why the washout numbers are a strong clue rather than a verdict.
- Adrenocortical carcinoma: a large, heterogeneous mass with irregular enhancement, central necrosis, hemorrhage, and sometimes calcification; look hard at the renal vein and IVC for tumor thrombus and at the kidney and liver for direct invasion. Central necrosis, per the ACR, raises the likelihood of metastasis and is a reason to consider biopsy or PET/CT.
MRI
- Myelolipoma: the macroscopic fat is bright on T1 and follows fat on every sequence — it drops out on fat-saturated images. That is different from an adenoma's chemical-shift dropout, which is loss on opposed-phase compared with in-phase images and reflects fat and water sharing a voxel.
- Pheochromocytoma: 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. Avid enhancement after gadolinium, frequently with cystic areas. No signal loss on opposed-phase images, because there's no intracellular lipid.
- Adrenocortical carcinoma: heterogeneous on both T1 and T2, with hemorrhage (T1-bright) and necrosis (T2-bright, non-enhancing). MRI is the better tool for mapping venous tumor thrombus and the interface with the liver and kidney.
Nuclear medicine
When a pheo lives outside the adrenal or you need to hunt for hidden or multiple ones, functional imaging steps in — see the adrenal MIBG scan, which targets catecholamine-handling tissue specifically. FDG PET/CT is the problem-solver for a suspected ACC or metastasis, and the ACR floats it (alongside biopsy) when a mass shows central necrosis. Myelolipoma needs no nuclear study at all.
Ultrasound
Ultrasound is usually the bystander that found the mass. A myelolipoma is strikingly echogenic (fat); a pheo is a solid, often heterogeneous mass; an ACC is a large heterogeneous mass. None of these is characterized with confidence on ultrasound, and every one goes on to CT or MRI.
The numbers
| What | Threshold / value | Why it matters |
|---|---|---|
| Macroscopic fat in an adrenal mass | Present = myelolipoma | A diagnostic benign feature; no additional workup or follow-up imaging |
| Isolated adrenal mass ≥4 cm, no benign diagnostic features, no cancer history | Surgical resection without biopsy | The size at which possible adrenocortical carcinoma is treated rather than watched |
| Indeterminate mass 1 to <4 cm and >10 HU | Dedicated adrenal-protocol CT | The workup step that decides between adenoma and "something else" |
| Unenhanced attenuation | ≤10 HU homogeneous = lipid-rich adenoma | Pheo and ACC don't pass this test; a mass that does isn't either of them |
| Absolute washout | ≥60% favors adenoma | The pheo pitfall: a pheochromocytoma can occasionally clear this bar, so a positive biochemistry trumps a "benign" washout |
| Central necrosis | Raises the likelihood of metastasis | Consider biopsy or PET/CT |
| Growth rate separating benign from malignant | None established | Both benign and malignant masses may enlarge; an enlarging indeterminate mass goes to biochemical evaluation and, depending on growth, resection |
Size and growth matter, but not as a formula. There's no growth-rate cutoff that proves cancer — the ACR white paper says so explicitly. What there is, is the 4 cm rule: an isolated mass at or above it with nothing benign about it earns a surgeon, not a follow-up scan.
How good is the test
For the specific question this page asks — is it a myelolipoma, a pheo, or an ACC — there are no robust pooled sensitivity or specificity figures, so I won't quote any. What we do have is categorical: the ACR classes macroscopic fat as a diagnostic benign feature, meaning CT alone settles the myelolipoma question, and the adenoma tests (≤10 HU unenhanced, washout ≥60%) are the yardstick against which these three are measured — a mass that clears them is almost never a pheo or an ACC, and a mass that doesn't has earned its biochemistry and its specialist.
The differential
| Mimic | Looks similar because | Tell them apart by |
|---|---|---|
| Lipid-rich adenoma (vs myelolipoma) | Both "contain fat" | Adenoma fat is microscopic: low average HU (≤10) or opposed-phase dropout; myelolipoma fat is macroscopic pockets at fat attenuation |
| Retroperitoneal liposarcoma (vs myelolipoma) | Fat-containing retroperitoneal mass | Arises outside the gland and displaces it; large, with thick septa or nodular soft tissue; the adrenal is separately identifiable |
| Renal angiomyolipoma (vs myelolipoma) | Fat-containing mass in the same neighborhood | Claw sign from the kidney; the adrenal is seen separately |
| Lipid-poor adenoma (vs pheo) | Both are >10 HU and enhance | Adenoma washes out (≥60% absolute); pheo usually doesn't, and the metanephrines are normal |
| Metastasis (vs pheo or ACC) | Enhancing, may be necrotic | Known primary, often bilateral; biochemistry negative; biopsy once pheo is excluded |
| Adrenal hemorrhage (vs ACC) | Large heterogeneous adrenal mass | Dense unenhanced, no enhancement (<10 HU change), shrinks on follow-up; trauma or anticoagulation in the story |
| Upper-pole renal cell carcinoma (vs ACC) | Large heterogeneous mass near the adrenal | Claw sign with the kidney; coronal and sagittal reformats show the organ of origin |
Reporting
- Organ of origin: say explicitly that the mass arises from the adrenal and the adrenal is not seen separately (or that it is, which points elsewhere).
- Size in the largest dimension, flagged against the 4 cm line.
- Composition: macroscopic fat present or absent; unenhanced attenuation in HU; enhancement pattern; necrosis, hemorrhage, calcification.
- Extent: invasion of the kidney or liver, tumor thrombus in the adrenal or renal vein and IVC, nodal or distant disease.
- Comparison and growth, without pretending a growth rate proves anything.
- The recommendation: for a fat-containing mass, "myelolipoma; no follow-up." For a mass with pheo features or any indeterminate mass in a hypertensive patient, biochemical evaluation before any biopsy. For an isolated mass ≥4 cm with nothing benign about it, surgical referral.
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, order metanephrines, and keep the needle away until you know. Big, heterogeneous, invading the neighbors, at or over 4 cm? Adrenocortical carcinoma — sound the alarm. Match the picture to the character and the adrenal gland stops being mysterious.
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: macroscopic fat as a diagnostic benign feature (no workup or follow-up), the ≥4 cm resection-without-biopsy rule, the 1 to <4 cm adrenal-protocol CT rule, the ≤10 HU unenhanced adenoma threshold, the ≥60% absolute washout cutoff, the <10 HU non-enhancing definition, the central-necrosis guidance, and the "no growth-rate threshold" statement in "Findings by modality," "The numbers," "How good is the test," "The differential," 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 as the adenoma yardstick in "The numbers" and "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 that separates microscopic adenoma fat from macroscopic myelolipoma fat in "Findings by modality"; no numeric index is taken from it.
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