Adrenal (MIBG)
- MIBG is a molecular decoy that looks like norepinephrine, so cells that hoard catecholamines greedily suck it up and light up on the scan.
- It hunts the adrenal medulla's troublemakers: pheochromocytoma, paraganglioma, and the kid-world villain, neuroblastoma.
- I-123 is the diagnostic workhorse (cleaner pictures); I-131 is heavier-hitting and used more for therapy.
- Block the thyroid first. The radioactive iodine riding on MIBG will happily wander off and irradiate a gland you weren't aiming at.
- Whole-body imaging is the point: MIBG finds disease anywhere it hides, not just the one lump CT already saw.
Imagine you're trying to find every coffee addict in a city. You could search house by house (that's CT, slow and local), or you could spike the water supply with a harmless glowing coffee look-alike and just watch who lights up. The caffeine fiends grab it, the rest ignore it. That second trick is basically MIBG — a glowing decoy that only the catecholamine-obsessed cells of the body can't resist.
What the molecule actually is
MIBG stands for metaiodobenzylguanidine, which is a mouthful engineered to sound important. The useful part: its shape is close enough to norepinephrine (one of the body's "fight or flight" hormones) that the cellular machinery built to grab and store norepinephrine grabs MIBG by mistake. We bolt a radioactive iodine atom onto it, and now wherever the molecule gets hoarded, a gamma camera can see the glow.
So MIBG isn't really an "adrenal scan" in the geographic sense — it's a behavioral scan. It finds cells that behave like adrenal medulla cells, wherever they happen to be living.
MIBG images function, not anatomy. A tumor lights up because of what it does (hoards catecholamines), not where it sits. That's why it can find disease CT completely missed.
What we're hunting
The adrenal medulla — the core of the adrenal gland — is the body's little catecholamine factory. When that factory goes rogue, you get the classic MIBG targets:
| Tumor | Who gets it | The MIBG angle |
|---|---|---|
| Pheochromocytoma | Adults | Adrenal medulla tumor pumping out catecholamines; sweats, palpitations, blood pressure that spikes like a startled cat. |
| Paraganglioma | Adults | Same cell type, but living outside the adrenal (anywhere along the sympathetic chain). The "extra-adrenal pheo." |
| Neuroblastoma | Young children | The classic pediatric solid tumor of sympathetic tissue; MIBG is central to staging and follow-up. |
The first mention earns links: pheochromocytoma is the adult adrenal lesion you stage and survey, and neuroblastoma is where MIBG truly shines in kids.
I-123 versus I-131: same decoy, different luggage
The MIBG decoy can carry one of two radioactive iodine isotopes, and which one matters a lot.
- I-123 MIBG is the diagnostic favorite. It gives crisp images, a friendlier radiation dose, and pairs nicely with SPECT/CT for pinpointing where the glow actually sits. Think of it as the good camera.
- I-131 MIBG emits a heavier, more damaging type of radiation. That makes it grainier for pictures but useful as a treatment — the same molecule that finds the tumor can deliver a radioactive payload to kill it. The decoy becomes a guided missile.
This "find it, then treat it with the same molecule" idea is the heart of theranostics. For the neuroendocrine cousins that prefer somatostatin receptors over catecholamine pumps, the analogous tool is DOTATATE imaging and therapy.
Don't poison the thyroid (the prep that matters)
Here's the catch with bolting radioactive iodine onto anything: the thyroid loves iodine more than almost any organ in the body. If the iodine atom falls off the MIBG molecule and floats free, the thyroid will scoop it up and absorb a dose meant for nobody.
So before the scan, we block the thyroid with stable (non-radioactive) iodine — usually potassium iodide — to fill those parking spots so the radioactive stragglers have nowhere to land. This is the same logic as the I-131 thyroid therapy world, just running in reverse: there we want the thyroid to eat iodine; here we're desperately keeping it away.
A long list of common drugs — certain antidepressants, decongestants, some blood-pressure agents like labetalol — interfere with how cells take up MIBG and can produce a falsely negative scan. The patient may need to pause these beforehand. If a known catecholamine tumor "doesn't light up," suspect medication interference before declaring the scan clean.
Reading it without getting fooled
The trap with MIBG is that normal things glow too. The liver, salivary glands, heart muscle, and bladder all show physiologic uptake, and the bowel can be unpredictable. Your job is to separate "that's supposed to be bright" from "that bright spot is a problem."
SPECT/CT is the great clarifier. Planar MIBG tells you something lights up; fusing it with CT tells you exactly which anatomic structure is glowing — adrenal nodule versus a loop of bowel versus a rib met. When a focus is ambiguous, the answer is almost always "get the SPECT/CT."
The one thing to walk away with
MIBG turns a hard anatomic question — where is all the disease? — into an easy behavioral one. By disguising a radioactive tag as a fight-or-flight hormone, it makes catecholamine-hoarding tumors announce themselves wherever they're hiding. Prep the thyroid, mind the interfering meds, reach for SPECT/CT when in doubt, and remember that the same decoy that finds the tumor can, in its I-131 form, be turned around to treat it.