“Cystic-Appearing” Posterior Fossa Lesion

Susan I. Blaser, MD, FRCPC

DIFFERENTIAL DIAGNOSIS

Common

Mega Cisterna Magna
Arachnoid Cyst
Dandy-Walker Continuum
Pilocytic Astrocytoma
Encephaloceles
Obstructive Hydrocephalus

Less Common

Epidermoid Cyst
Dermoid Cyst
Neuroglial Cyst
Ependymal Cyst
Hemangioblastoma
Schwannoma (Cystic)
Abscess
Enlarged Perivascular Spaces

Rare but Important

Syringobulbia
Neurenteric Cyst
Atypical Teratoid-Rhabdoid Tumor
Metastases, Intracranial, Other
Neurocysticercosis
Chordoma
Congenital Muscular Dystrophy

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Cystic-appearing lesion exactly like CSF on all sequences?
- Mega cisterna magna (MCM), arachnoid cyst (AC), Dandy-Walker Continuum (DW)
- Trapped 4th ventricle, enlarged perivascular spaces (↑ PVSs), neuroglial or ependymal cyst
Cystic-appearing lesion not exactly like CSF?
- Congenital inclusion cyst (dermoid, epidermoid, neurenteric cysts)
- Infection such as abscess, neurocysticercosis (NCC)
- Neoplasm (pilocytic astrocytoma, hemangioblastoma, metastasis, chordoma)
Is cyst intra- or extra-axial?
Intra-axial
- Trapped fourth ventricle (4th V), ↑ PVSs
- Neoplasm (e.g., pilocytic astrocytoma), infection (abscess, NCC)
- Inclusion cyst in 4th V (epidermoid)
Extra-axial
- MCM, AC, DW, neurenteric cyst, NCC, neoplasm (schwannoma)
DWI, T1 C+ scans helpful additions

Helpful Clues for Common Diagnoses

Mega Cisterna Magna
- Communicates freely with all CSF spaces
- Normal tegmento-vermian angle (< 5-10°)
Arachnoid Cyst
- Mass effect on vermis
- ± Hydrocephalus
- Use FLAIR, DWI to exclude epidermoid
Dandy-Walker Continuum
- “Classic” Dandy-Walker malformation
  - Cystic dilatation 4th V ⇒ ↑ posterior fossa (PF), torcular-lambdoid inversion
  - Hypoplastic vermis
  - Vermian remnant rotated anterosuperiorly over cyst
- Blake pouch cyst (BPC)
  - Embryonic BPC doesn’t regress
  - Enlarged PF, 4th V open inferiorly
  - Vermis anatomically complete
Pilocytic Astrocytoma
- Cystic cerebellar mass
- Enhancing mural nodule
Encephaloceles
- Isolated encephalocele: Lacks Chiari 2
- Chiari 3 = Chiari 2 PLUS
  - Occipital or cervical encephalocele containing cerebellum
- Syndromic occipital encephalocele
  - Klippel-Feil, Meckel-Gruber, etc.
Obstructive Hydrocephalus
- Outlets obstructed→ 4th ventricle ↑ ↑
- Maintains “kidney bean” configuration
- 3rd V, shunted lateral ventricles small

Helpful Clues for Less Common Diagnoses

Epidermoid Cyst
- Cerebellopontine angle > 4th V > diploic
- Frond-like, cystic (CSF-like)
- Doesn’t suppress completely on FLAIR
- Restricts on DWI
Dermoid Cyst
- Midline “fatty” mass
  - “Droplets” in CSF if ruptured
  - Look for dermal sinus, midline vertebral/skull base anomalies
Neuroglial Cyst
- CSF-like parenchymal cyst
- No enhancement, DWI restriction
Ependymal Cyst
- CSF-like
- Intra- > paraventricular
Hemangioblastoma
- Posterior fossa mass with cyst, enhancing mural nodule that abuts pia
- ± Arterial feeders, flow-voids
- Look for markers of von Hippel-Lindau (VHL)
  - Visceral cysts, renal clear cell carcinoma
- Adult > > older teen (unless VHL)
  - Check family history!
Schwannoma (Cystic)
- Vestibular schwannoma (VS) looks like “ice cream on cone”
- Cysts can be intratumoral or VS-associated (arachnoid)
- Solid component enhances
Abscess
- T2 hypointense rim with surrounding edema
- Ring-enhancing
- DWI hyperintense, ADC hypointense
Enlarged Perivascular Spaces
- CSF-like, nonenhancing, nonrestricting
- Most common PF site = dentate nuclei
- Less common = cerebellum, pons

Helpful Clues for Rare Diagnoses

Syringobulbia
- May occur with either Chiari 1 or 2
- Cervical/holocord syrinx common
- May extend further into brain (syringocephaly)
Neurenteric Cyst
- Slightly hyperintense extra-axial cystic mass, nonenhancing
- Anterior pontomedullary, CPA cisterns
Atypical Teratoid-Rhabdoid Tumor
- 50% infratentorial (usually off-midline)
- Intratumoral cysts, hemorrhage common
- Gross macrocysts less common
Metastases, Intracranial, Other
- Myriad of nonenhancing interfoliate cysts
  - Low or high grade brain or spine primary
  - Also reported with breast primary
- Choroid plexus papilloma cysts can be entirely extra-axial, nonenhancing
Neurocysticercosis
- Cyst with “dot” (scolex) inside
- Subarachnoid spaces, sulcal depths most common
- Intraventricular cysts often isolated
  - 4th ventricle most common
Chordoma
- High signal T2
- Moderate to marked enhancement unless necrotic, mucinous
- High attenuation foci (CT) may be occult on MR
Congenital Muscular Dystrophy
- Best diagnostic clues
  - Severely “floppy” infant
  - Z-shaped or cleft pons
  - Multiple small CSF-like cerebellar cysts (may be PVSs or trapped CSF from overmigration of neurons)

Image Gallery

Sagittal T1WI MR shows a mega cisterna magna

. The tentorium is normally located, and the posterior fossa is mildly prominent. There is no mass effect upon the vermis.

Sagittal T1WI MR shows a retrocerebellar arachnoid cyst. There is enlargement of the posterior fossa, elevation of the tent, and mild compression of the vermis.

(Left) Sagittal T1WI MR shows typical enlarged posterior fossa, upward rotation of the small vermian remnant, elevation of the tentorium, and mass effect upon the brainstem in “classic” Dandy-Walker malformation. (Right) Sagittal T2WI MR shows enlargement of the inferior 4th ventricle