Aneurysmal Bone Cyst (ABC)
- An aneurysmal bone cyst (ABC) is a benign, blood-filled, ballooning lesion of bone — not a true cyst and not actually an aneurysm, just a name that overpromises on both counts.
- It loves kids and young adults (typically under 20-ish) and classically sits in the metaphysis of long bones or the posterior elements of the spine.
- The signature finding is an expansile, lytic lesion with fluid–fluid levels on MRI, often with a thin shell of bone bulging the cortex outward.
- It is benign, but it can be locally aggressive and can recur — and other lesions can wear its costume, so it's a diagnosis you confirm, not assume.
Imagine a balloon being slowly inflated inside the wall of a bone. The bone doesn't burst — it grudgingly stretches, thinning its cortex into a fragile eggshell while the balloon fills with little compartments of blood. That, in one mental picture, is an aneurysmal bone cyst. The name is a beautiful piece of medical false advertising: it's not an aneurysm (no arterial wall, no pulsation) and it's not really a cyst (no simple watery sac — it's a sponge of blood-filled chambers). But the name stuck, so here we are.
What it actually is
Under the microscope, an ABC is a bunch of blood-filled spaces separated by fibrous walls — think of a honeycomb that someone filled with blood instead of honey. It's benign. It doesn't metastasize. But it can be locally aggressive: it expands, it remodels bone, and after treatment it has an annoying habit of coming back.
Here's the wrinkle that matters for accuracy: an ABC can arise on its own (primary), or it can grow on top of another bone lesion that set the whole process in motion (secondary). That second flavor is the reason you can't just see fluid–fluid levels and call it a day — something else might be hiding underneath.
"Aneurysmal" here refers to the ballooned, blown-out shape of the bone, not to a blood vessel. The lesion expands the bone like a localized aneurysm expands a vessel — same silhouette, completely different plumbing.
Who gets it and where
ABCs are mostly a young person's lesion — they skew toward children and adolescents, classically before the growth plates close. Location is one of your best clues:
| Site | Typical spot |
|---|---|
| Long bones (femur, tibia, humerus) | Metaphysis, often eccentric (off to one side) |
| Spine | Posterior elements (the bony arch behind the cord), sometimes spilling into the body |
| Flat bones | Pelvis is a known hangout |
That metaphyseal, eccentric, long-bone pattern in a teenager is the textbook setup. When you see an expansile lesion in the posterior elements of the spine in a young patient, ABC should be near the top of your list.
What it looks like on imaging
On a plain radiograph, an ABC is a lytic (dark, bone-eaten) lesion that expands the bone — the cortex bulges outward and thins into that eggshell rim I mentioned. It often looks well-defined, like the bone is trying to wall it off, but it can blow the cortex out far enough to look alarming.
The money shot is on MRI, and the word to tattoo on your memory is fluid–fluid levels. Because the lesion is full of blood that has had time to settle in the bore of the scanner, the heavier cells sink and the lighter serum floats — like a vinaigrette left on the counter, oil on top, vinegar below. On MRI you see multiple little compartments, each with a flat line separating a brighter top from a darker bottom. A thin, dark septated rim usually surrounds the whole thing.
CT confirms the expansile, thin-shelled architecture and can also show fluid–fluid levels; it's especially handy in the spine and pelvis where overlapping bone makes radiographs muddy.
Fluid–fluid levels are the ABC calling card — but they are not exclusive to it. They reflect settled blood, which can show up in other lesions too.
The traps that will get you
This is the part that keeps radiologists honest. Fluid–fluid levels feel like a smoking gun, but several lesions can produce them.
The dangerous mimic is telangiectatic osteosarcoma — a malignant tumor that can be cystic and blood-filled enough to also show fluid–fluid levels and masquerade as a benign ABC. The tells that should worry you: a soft-tissue mass, destroyed/permeated cortex, an aggressive periosteal reaction, or nodular/thick enhancing tissue rather than thin septa. When the picture looks "ABC but angrier," push for biopsy.
A couple more to keep on the radar:
- Secondary ABC riding on another lesion — most famously a giant cell tumor, which can develop ABC-like blood-filled areas. Always ask whether there's a solid component underneath.
- Simple (unicameral) bone cyst — also expansile and lytic, but it's a single watery cavity, typically central rather than eccentric, and it doesn't sport the blood-filled multi-compartment look.
This is exactly why even a "classic" ABC usually earns a closer look before treatment: the consequences of mistaking a telangiectatic osteosarcoma for a benign cyst are not the kind you get to walk back.
The one thing to remember
An ABC is a benign, blood-filled, expansile lesion in a young patient — eccentric metaphysis of a long bone or posterior spine — and its signature on MRI is fluid–fluid levels inside a thin-shelled, ballooned bone. Love the pattern, but never trust it blindly: confirm that nothing malignant is wearing the costume underneath.