Pediatric Brain Tumors
- In kids, brain tumors love the posterior fossa — the basement crawlspace where the cerebellum and brainstem live — which is the opposite of adults, who tend to grow tumors up top.
- The big three midline-ish posterior fossa players are medulloblastoma, pilocytic astrocytoma, and ependymoma, and you tell them apart mostly by where they sit, how dense/bright they are, and whether they squeeze through holes.
- A posterior fossa mass plugs the drainpipe, so the presenting picture is often obstructive hydrocephalus — the swollen ventricles, not the tumor, are what the kid feels first.
- Diffusion is your secret weapon: densely-packed "small round blue cell" tumors like medulloblastoma restrict diffusion (glow on DWI); the friendly cystic astrocytoma does not.
- MRI is the workhorse; the goal isn't a slam-dunk tissue diagnosis from imaging, it's a tight, surgically useful short list.
Here's the rule that flips your adult instincts upside down: in adults, brain tumors mostly set up shop above the tentorium (the tent of dura separating cerebrum from cerebellum). In children, a huge chunk of them move into the posterior fossa — the cramped basement under that tent, where the cerebellum and brainstem are jammed together with barely any elbow room. Think of the skull as a two-story house. Adults remodel the upstairs. Kids flood the basement.
And the basement is a terrible place to flood, because the brain's main drainpipe — the fourth ventricle and the narrow channels around it — runs right through it.
Why hydrocephalus steals the show
Cerebrospinal fluid (CSF) is made deep in the brain, flows down through a series of ever-narrower chambers, and exits near the bottom into the space around the brain and cord. A posterior fossa mass sits squarely on that exit and pinches it shut. Fluid keeps getting made; it just can't leave.
So the ventricles balloon — that's obstructive hydrocephalus — and the kid shows up with morning headaches, vomiting, and a wobbly walk long before anyone's thinking "tumor." On imaging, the dilated ventricles can be louder than the mass itself. Always chase the dilated fourth ventricle upstream to find what's blocking it.
In a young child the soft fontanelles and unfused sutures can absorb some of this pressure, so the classic adult crash of acute hydrocephalus may be muted — replaced by a head that's quietly growing too fast. A head circumference creeping up the wrong way on the growth chart is a real finding, not background noise.
The big three of the posterior fossa
Most of the time you're sorting between three tumors, and the trick is a short checklist: Where exactly? How dense or bright? Does it restrict diffusion? Does it ooze out through holes?
| Tumor | Typical home | Classic imaging feel | Diffusion |
|---|---|---|---|
| Medulloblastoma | Midline, roof of the 4th ventricle (vermis) | Dense/hyperdense on CT, often fairly solid, enhances | Restricts (densely packed cells) |
| Pilocytic astrocytoma | Cerebellar hemisphere, off to the side | Big cyst with an enhancing nodule on the wall | Does not restrict |
| Ependymoma | Inside the 4th ventricle | Squishes and molds, squeezes out the foramina like toothpaste | Variable |
That last column is the cheat code. Medulloblastoma is the classic "small round blue cell" tumor — cells crammed shoulder-to-shoulder with almost no space between them — and that crowding traps water molecules so they can't wander freely. On diffusion-weighted imaging the tumor lights up bright. The cystic pilocytic astrocytoma, by contrast, is mostly water and breathing room, so it stays dark. One sequence, half the differential.
The ependymoma's party trick is being a plastic tumor: it grows inside the fourth ventricle and, rather than respecting walls, oozes out through the little exit holes (the foramina) into the spaces around the brainstem. When you see a posterior fossa mass squeezing out of an opening like dough through a pasta press, ependymoma jumps up the list.
Don't forget the brainstem itself
Not every pediatric posterior fossa tumor is a discrete ball you can point at. Some diffuse gliomas infiltrate the brainstem itself — most notoriously expanding the pons — so the brainstem just looks swollen and abnormal on T2/FLAIR rather than showing a tidy enhancing mass. These are devastating and largely defined by where they grow and how they spread, not by a clean margin you can lasso.
Pediatric brain tumors are not exclusively a posterior fossa story. Plenty live above the tentorium — around the optic pathway and hypothalamus, in the pineal and suprasellar regions, or within the cerebral hemispheres. "Kid = posterior fossa" is a great starting bias, but a mass up top doesn't let a tumor off the hook.
Two habits that save you
First, image the whole neuraxis. Several of these tumors — medulloblastoma especially — like to seed CSF and drop metastases down the spine ("drop mets"). A brain MRI alone undersells the disease, so the spine gets imaged too, ideally before surgery stirs up blood that mimics tumor.
Second, resist the urge to over-call the tissue type from pixels. Imaging narrows the list and tells the surgeon what they're walking into; the actual diagnosis comes from the pathologist and, increasingly, molecular markers that imaging can't see. Modern classification of these tumors leans heavily on genetics — which is exactly why your job is a confident short list, not a one-word verdict.
When you spot a posterior fossa mass in a child, narrate three things out loud: its compartment (in the ventricle, on the roof, or off in a hemisphere), its diffusion behavior, and the state of the ventricles upstream. Those three answers do most of the heavy lifting before you've named anything.
If you want the grown-up counterpart to compare against — same biology, different neighborhoods — the gliomas page and your general approach to brain MRI are the natural next stops. The single thing to carry out of here: in a child with headaches and a swollen ventricular system, go looking in the basement.