Extradural Tumors & Metastases
- "Extradural" means the lesion lives outside the dura — in the bone or the epidural space — pushing the dura and cord inward rather than living inside the sac.
- This is the most common spine tumor compartment, and the headline act is metastatic disease to the vertebrae, not a primary tumor.
- The localizing trick: extradural lesions widen the bone/epidural space and squash the dura-cord unit away from the lesion (the cord stays in the middle, just shoved).
- MRI is the workhorse; the bone marrow does the talking on T1 — normal fatty marrow is bright, tumor replacing it goes dark.
- The reason any of this matters urgently is one phrase: it can crush the cord. Always look for compression.
The spine is built in layers, like one of those nesting dolls nobody can ever put back together. Way on the outside is bone; just inside that is a thin fatty sleeve called the epidural space; then a tough waterproof bag (the dura) holding the cord and its bath of cerebrospinal fluid. "Extradural" tumors are the ones that set up shop in those outermost layers — the bone and the epidural space — and never break into the bag. That single fact does almost all the diagnostic work for you.
The three-compartment game
Spine tumors are sorted by where they live, because where they live tells you what they probably are. There are three boxes: inside the cord (intramedullary), inside the bag but outside the cord (intradural-extramedullary), and outside the bag entirely — extradural. Extradural is the big, crowded box: it's the most common of the three, mostly because metastases love the vertebral bone.
The way you prove a lesion is extradural is by watching what it does to its neighbors. An extradural mass pushes the whole dura-and-cord unit inward, like a thumb pressing on the side of a water balloon. The cord drifts away from the lesion, and on the side facing the mass you often see the CSF column pinched off sharply.
Quick orientation trick: extradural lesions push the cord away and narrow the CSF on the lesion side. Intradural-extramedullary lesions sit inside the sac and tend to widen the CSF space around themselves (a "CSF cap"). Same mass effect, opposite signature.
Who actually shows up: metastases run the table
If you remember one thing, make it this: in an adult with a known cancer and a new vertebral lesion, the smart money is on metastasis until proven otherwise. The vertebral bodies are basically a hotel chain for tumor cells — rich red marrow with a generous blood supply. Breast, lung, and prostate are the usual guests; lymphoma, myeloma, and renal cell turn up plenty too. After metastases, the common-ish primary extradural players include the marrow-based diseases (myeloma) and a few benign bone tumors.
Mets come in two flavors, and the flavor depends on whether the tumor tells bone to dissolve or to pile up:
| Pattern | What the bone does | Classic offenders | On CT / radiograph |
|---|---|---|---|
| Lytic | Bone gets eaten away | Lung, kidney, thyroid, myeloma | Dark holes, lost cortex, a "missing" pedicle |
| Sclerotic (blastic) | Bone gets piled up | Prostate, treated breast | Dense white spots, sometimes "ivory" vertebra |
| Mixed | A bit of both | Breast (often) | Patchy, ugly, all-of-the-above |
Why MRI wins, and the one sequence that matters
You can see lytic and sclerotic mets on CT and even on plain radiographs — but a radiograph needs a big chunk of bone destroyed before it whispers, so it's a late, blunt tool. MRI sees the marrow itself, much earlier. (For the full walkthrough, see approach to spine MRI.)
The trick is the T1 sequence: normal adult marrow is full of fat, and fat is bright on T1. Tumor moves in, evicts the fat, and the marrow goes dark. So a focal lump of low T1 signal in a vertebral body that should be cheerfully bright is the money finding. A useful sanity check: tumor should be darker than the adjacent normal disc on T1.
Not every dark-on-T1 marrow spot is cancer. A recent fracture, infection, or even areas of red marrow can mimic it. The strongest reassurance that a vertebral collapse is benign (osteoporotic) rather than malignant is a preserved fatty marrow signal in the rest of the vertebra and an intact posterior cortex — when tumor causes the collapse, it usually leaves a soft-tissue mass and bulges the bone backward.
The thing you must never miss
Bone disease is one problem; the cord is the emergency. As an extradural mass grows, it can push backward into the epidural space and squeeze the dural sac shut. That's cord compression, and it's a true "now, not tomorrow" situation — every hour of compression is neurologic function you may not get back.
When you find one extradural met, image the whole spine. Metastases are rarely shy and rarely solitary, and a second, higher level of compression changes the whole plan.
So the whole page in one breath: extradural means outside the bag, it's the busiest compartment, metastasis is the default suspect, T1 marrow signal is your magnifying glass, and the cord is the reason you don't dawdle.