A tumor of the cells that line the ventricles. In kids, it parks in the 4th ventricle and chokes off CSF flow. In adults, it slides down into the spinal cord. The histology fingerprint is a circle of tumor cells hugging a blood vessel.
A 6-year-old boy is brought in for three weeks of morning headaches that wake him before breakfast and ease once he's been upright for an hour. This morning he had a forceful, projectile vomit with no nausea warning. On exam he is mildly ataxic and his head tilts toward the right. MRI shows a contrast-enhancing mass filling the 4th ventricle, with enlarged lateral and third ventricles upstream.
What's the most likely diagnosis?
4th ventricle mass in a kid + obstructive hydrocephalus = ependymoma until proven otherwise.
Medulloblastoma also lives in the posterior fossa, but it grows from the cerebellar vermis and pushes INTO the 4th ventricle from the roof rather than filling it from within. Pilocytic astrocytoma is the cystic cerebellar-hemisphere tumor with a mural nodule, not a midline ventricle-filling mass. Craniopharyngioma sits in the suprasellar region with calcifications and visual-field defects, nowhere near the 4th ventricle.
The morning headache + projectile vomit is the classic intracranial pressure pattern: CSF backs up overnight while the kid is supine, pressure peaks at wake-up, and the medulla's vomiting center fires from the back of the brainstem (no nausea warning because it's a brain reflex, not a gut one).
Step 1 · Spot the Pattern
The Four Clues That Crack It
Every board ependymoma question hides at least one of these. Spot any of them in a stem and you're done thinking.
🩸Histology Fingerprint
Perivascular Pseudorosette
Tumor cells arranged in a ring around a central blood vessel, with their pink (eosinophilic) fibrillary tails pointing inward toward the vessel. Looks like a daisy with the petals pointing IN. This is the pathognomonic finding.
True rosettes (a ring around an empty central lumen) can also appear but are rarer. The PERIVASCULAR one wins every time.
🧠Location · Kid
4th Ventricle
In children, ependymoma's home base is the 4th ventricle, sitting in the posterior fossa between the brainstem and cerebellum. The tumor literally fills the ventricle from inside.
Posterior fossa real estate in kids: medulloblastoma takes the cerebellar vermis (above the 4th), pilocytic takes the cerebellar hemispheres (lateral), ependymoma takes the 4th itself.
🧬Cell of Origin
Ependymal Cells
The ependyma is the single layer of cells that lines the entire ventricular system and the central canal of the spinal cord. Anywhere there's ependyma, you can grow an ependymoma. That's why the location shifts with age.
In adults, the same cell type makes spinal-cord ependymomas (most common in the cauda equina region, the myxopapillary subtype).
💧Why The Symptoms
Obstructive Hydrocephalus
A tumor parked in the 4th ventricle blocks the only exit for CSF coming down from above. CSF backs up into the third and lateral ventricles. Pressure climbs, headache hits, brainstem vomiting center fires.
Morning headache + projectile vomit + ataxia in a kid is the classic triad. Imaging confirms with ballooned upstream ventricles.
Step 2 · Anatomy
Where The Tumor Lives
CSF flows top to bottom: lateral ventricles → third → cerebral aqueduct → 4th ventricle → spinal central canal. Ependymal cells line every wall of this system. Tap the age toggle to see where ependymoma chooses to grow in each population.
CSF Highway
Sagittal mid-line view · ependyma is the lining
Pediatric: ependymoma plants itself in the 4th ventricle (posterior fossa, infratentorial). It physically blocks CSF outflow through the foramina of Luschka and Magendie. Result: ballooned lateral and third ventricles upstream, morning headache, projectile vomiting, ataxia.
Adult: ependymoma shifts down the highway. Spinal cord becomes the most common site, especially the central canal in the cauda equina region (myxopapillary subtype). Presents as chronic lower back pain, saddle anesthesia, leg weakness, bladder dysfunction.
Step 3 · The Slide
Histology · Perivascular Pseudorosette
A real H&E slide on the left, a drawn schematic on the right. The schematic shows exactly what your eye should be tracking: a central blood vessel surrounded by tumor cells whose pink fibrillary processes point inward like the rays of a flower.
Real Slide · H&E
Intermediate magnification H&E. Look for the pale fibrillary haloes around small blood vessels · that nuclear-free pink ring IS the pseudorosette.
Tap to enlarge
Wikimedia Commons · CC BY-SA 3.0 (Nephron)
Schematic · What You're Looking At
Same scene as the slide, idealized. Pink rays = the cytoplasmic processes that reach toward the vessel. That nucleus-free perivascular ring is the giveaway.
Schematic · original, drawn beside the slide
The Chain: ependymal cells (which line ventricles and have processes reaching toward blood vessels for nutrients) become cancerous → they keep their original architecture → on a slide they huddle around the nearest vessel with their tails pointing in → that radial pink halo IS the diagnosis. No other CNS tumor does this.
Step 4 · Don't Confuse It With
The Three Tumors That Look Similar
All four can be a kid + headache + brain mass. The histology fingerprint is the discriminator. One look at the slide and you're done.
Pilocytic Astrocytoma
The Cyst With A Bump
Most common pediatric brain tumor · benign
LocationCerebellar hemisphere (lateral, not midline). Cystic mass with a mural nodule.
HistologyRosenthal fibers · corkscrew-shaped pink protein deposits. Eosinophilic granular bodies.
PrognosisExcellent. Resection is usually curative.
LocationCerebellar vermis (midline, but above the 4th ventricle · pushes down into it from the roof).
HistologyHomer Wright rosettes · tumor cells around a CENTRAL TANGLE of pink fibrils (no blood vessel).
PrognosisAggressive, small round blue cell tumor, can drop CSF metastases down the spinal cord.
Homer Wright = empty fibrillary center, NO vessel. Perivascular pseudorosette = central vessel WITH fibrils pointing at it. The vessel is the tell.
Craniopharyngioma
The Oily Motor-Oil Cyst
Suprasellar, derived from Rathke pouch remnants
LocationSuprasellar (above the pituitary, not in the ventricles at all). Compresses the optic chiasm.
HistologyCyst filled with motor-oil-like brown fluid loaded with cholesterol crystals. Calcifications on CT.
PresentationBitemporal hemianopsia, growth failure, hypopituitarism. Not posterior fossa.
Suprasellar + calcifications + visual field defect = craniopharyngioma. Different neighborhood entirely · nowhere near the 4th ventricle.
Step 5 · Lock It In
Quiz · Five Vignettes
Five clinical questions written for this page. Read the stem, scan the choices bottom-up, pick. Explanations show why the trap felt right and how to never fall for it again.
Medically reviewed by Kaitlyn Cocuzzo, MD and Fatima Ali, DO · Last updated June 30, 2026 at 1:10 AM ET
Bone Wizardry is an independent educational resource for visual learning in the medical sciences. It is not affiliated with, endorsed by, or sponsored by any licensing or examination board, contains no real or recalled examination questions, and does not guarantee any educational or examination outcome.
