Posterior Fossa Tumors
- The posterior fossa is the cramped basement under the brain, holding the cerebellum and brainstem — a mass here has nowhere to hide and quickly blocks the fourth ventricle, causing hydrocephalus.
- In adults, the most common posterior fossa masses are metastases and hemangioblastoma; in kids it's a different cast entirely (medulloblastoma, pilocytic astrocytoma, ependymoma, brainstem glioma).
- The two questions that crack most of these: where exactly is it (midline cerebellum, hemisphere, fourth ventricle, brainstem), and does it restrict on diffusion (packed-cell, "blue" tumors do).
- A child with morning headaches, vomiting, and an unsteady gait until proven otherwise has a posterior fossa tumor — image the whole neuraxis before surgery.
The posterior fossa is the architectural equivalent of the apartment under the stairs: small, oddly shaped, and packed with things you really can't afford to crush. The cerebellum and brainstem live here, and a single drainpipe — the fourth ventricle — carries cerebrospinal fluid (CSF) through the middle. Put a tumor in this room and two bad things happen almost immediately: vital brainstem real estate gets squeezed, and that drainpipe clogs, backing CSF up into the rest of the brain. So unlike a tumor in the roomy frontal lobe, a posterior fossa mass announces itself early and loudly.
Why location is the whole game
Before you even think about the tumor's name, ask where it sits. The posterior fossa rewards this more than almost anywhere in the brain, because each compartment has its own short list of usual suspects.
| Where it sits | Think first (adult) | Think first (child) |
|---|---|---|
| Cerebellar hemisphere | Metastasis, hemangioblastoma | Pilocytic astrocytoma |
| Midline / vermis, filling 4th ventricle | Metastasis | Medulloblastoma |
| Inside the 4th ventricle, oozing out the outlets | (less common) | Ependymoma |
| Brainstem (pons) | (less common) | Diffuse midline / brainstem glioma |
| Cerebellopontine angle cistern | Vestibular schwannoma, meningioma | (uncommon) |
That single column shift — adult versus child — is doing enormous work. Age is the most powerful filter you have here, so always pin it down first.
Old radiology adage worth internalizing: in an adult, a posterior fossa mass is a metastasis until proven otherwise — so go hunting for a primary and look for other lesions. In a child, the posterior fossa is where brain tumors live; a large share of pediatric brain tumors are infratentorial.
The pediatric headliners
Pediatric brain tumors cluster down here, and four names cover most of them.
Medulloblastoma is the midline troublemaker — it grows from the vermis and stuffs the fourth ventricle like an over-inflated balloon in a doorway. The key trick: it's a small-round-blue-cell tumor, meaning the cells are packed shoulder-to-shoulder with almost no room between them. Densely packed cells trap water, so it's dense on CT and restricts diffusion on MRI (bright on DWI). It also loves to seed CSF, dropping metastases down the spine — which is exactly why you image the whole neuraxis.
Pilocytic astrocytoma is the friendly one (relatively). Classic look: a big cyst with a single enhancing nodule on its wall, sitting in a cerebellar hemisphere. Picture a water balloon with one grape stuck to the inside — that grape is the bit that lights up with contrast. Slow-growing and often curable with surgery.
Ependymoma arises from the lining of the fourth ventricle and behaves like soft taffy: it squeezes out through the ventricle's exit holes (the foramina) into the adjacent cisterns. That "plastic," extruding-through-the-outlets morphology is the giveaway. (More on this one on the ependymoma page.)
Brainstem (diffuse midline) glioma expands the pons from within, often engulfing the basilar artery rather than displacing it. It's the heartbreaker of the group — infiltrative and difficult to treat.
Medulloblastoma and ependymoma can look similar — both are midline posterior fossa masses in a young child. Lean on behavior: medulloblastoma displaces the fourth ventricle and restricts diffusion strongly; ependymoma originates inside the fourth ventricle and characteristically extrudes through its outlet foramina. When stuck, the pattern of where it came from beats any single signal intensity.
The adult headliners
In grown-ups the math flips. The single most common intra-axial posterior fossa mass is a metastasis, so the moment you see a cerebellar lesion in an adult, your reflex should be to count: is there one, or are there several? And is there a known primary? See brain metastases for the full workup.
The classic adult primary down here is hemangioblastoma — a cyst-with-nodule lesion that mimics pilocytic astrocytoma but in an older patient, with a few telltales: the enhancing nodule abuts a pial (brain-surface) margin and may have prominent feeding vessels (flow voids). Multiple hemangioblastomas should make you think of the associated syndrome. The full story lives on the hemangioblastoma page.
The clinical thread to never drop
A posterior fossa mass plus an obstructed fourth ventricle equals obstructive hydrocephalus — and the crowded fossa leaves no give. The dangerous endpoint is tonsillar herniation, the cerebellar tonsils squeezed down through the foramen magnum like toothpaste, compressing the brainstem. This is the emergency hiding behind "just a headache." Look at the fourth ventricle and the size of the ventricles upstream on every one of these.
If you remember only one thing: in the posterior fossa, location plus age gets you most of the way to the diagnosis, and the fourth ventricle is the thing that turns an interesting tumor into a crisis. Name the compartment, name the patient's age group, and check the drainpipe — in that order.