Prepontine Cistern Mass

Gregory L. Katzman, MD, MBA

DIFFERENTIAL DIAGNOSIS

Common

CSF Flow Artifact
Dolichoectasia (Vertebrobasilar)
Fusiform Aneurysm, ASVD
Meningioma
Metastases, Skull and Meningeal

Less Common

Epidermoid Cyst
Chiari 2 (“Creeping Cerebellum”)
Exophytic Brainstem Glioma, Pediatric
Pituitary Macroadenoma (Giant)
Neurocysticercosis
Intracranial Hypotension

Rare but Important

Inflammatory Mass
- Tuberculosis
- Fungal Diseases
- Neurosarcoid
Clival Neoplasms
- Chordoma, Clivus
- Chondrosarcoma, Skull Base
- Plasmacytoma, Skull Base
- Nasopharyngeal Tumor (Invading Clivus)
Schwannoma
Arachnoid Cyst
Craniopharyngioma
Neurenteric Cyst
Ecchordosis Physaliphora

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Anatomy
- Extensive CSF space along ventral & lateral pons, dorsal to clivus (a.k.a, pontine cistern)
- Bounded superiorly by interpeduncular cistern, inferiorly by subarachnoid space of spinal cord, & continuous about medulla with cerebellomedullary cistern
Many abnormalities, often from transpatial processes

Helpful Clues for Common Diagnoses

CSF Flow Artifact
- MR artifacts divided into 2 categories: Time-of-flight effects & turbulent flow
- Worsens with thinner slices, longer TE, and imaging perpendicular to flow
- Assess real vs. artifact in other planes
- Minimize TOF losses: Use short TE, image parallel to flow, acquire thicker slices
Dolichoectasia (Vertebrobasilar)
- Older patients
- Look for ASVD in other vessels
- Ectasia often extends into branches
- May have significant mass effect on pons
Fusiform Aneurysm, ASVD
- Long segment fusiform arterial dilatation
- Involves long nonbranching segments
- Calcifications common
- Lumen enhances strongly, clot does not
Meningioma
- Clival dural-based enhancing mass
- Infratentorial (8-10%): CPA most common
- Causes cranial neuropathies or ataxia
Metastases, Skull and Meningeal
- Enhancing lesion(s) with skull/meningeal destruction/infiltration
- Manifestations: Smooth thickening, nodularity, loculation, fungating masses
- Image entire neuraxis!

Helpful Clues for Less Common Diagnoses

Epidermoid Cyst
- Usually extends medially from CPA cistern
- Lobulated, irregular, insinuating CSF-like mass
- Doesn’t completely suppress on FLAIR; restricts on DWI
Chiari 2 (“Creeping Cerebellum”)
- Small posterior fossa with low torcular herophili
- Cerebellar hemispheres/tonsils herniate anteriorly → “creeping”
- Pons, cranial nerve roots often elongated
Exophytic Brainstem Glioma, Pediatric
- Nonenhancing mass markedly expanding pons; may engulf basilar artery
- Infiltrative have poor survival
- Focal are uncommon, better prognosis
Pituitary Macroadenoma (Giant)
- No distinct pituitary gland
- Bone CT shows benign bony margins
- Early intense but heterogeneous CTST+
- Dural “tail” may mimic meningioma
Neurocysticercosis
- Cisterns > parenchyma > ventricles
- Basal cistern cysts may be racemose
- Cysts variable, typically 1 cm, range from 5-20 mm, contain a 1-4 mm scolex
- Most are isointense to CSF
Intracranial Hypotension
- Sagittal shows brain descent in 40-50%
- Pons may be compressed against clivus
- Diffusely, intensely enhancing dura in 85%
- Bilateral subdural fluid collections in 15%

Helpful Clues for Rare Diagnoses

Inflammatory Mass
- Tuberculosis
  - Basilar meningitis, pulmonary TB
  - Thick basilar exudate ± tuberculomas/abscesses
- Fungal Diseases
  - Blastomycosis, coccidiomycosis, histoplasmosis, candidiasis
  - Meningeal enhancement, multiple enhancing brain lesions
- Neurosarcoid
  - Classically infiltrates dura, leptomeninges, basal cisterns
  - Solitary or multifocal CNS mass(es) ± abnormal CXR
Clival Neoplasms
- Chordoma, Clivus
  - Destructive midline mass centered in clivus with high T2 signal intensity
  - Sagittal images show tumor “thumb” indenting anterior pons
- Chondrosarcoma, Skull Base
  - Arises from petro-occipital fissure
  - May extend posteriorly into prepontine cistern
  - Hyperintense on T2WI, enhances strongly but heterogeneously
  - Chondroid mineralization on CT (50%)
- Plasmacytoma, Skull Base
  - Solitary intraosseous osteolytic soft tissue mass with non-sclerotic margins
  - Peripherally displaced osseous expansion/fragmentation may be seen
- Nasopharyngeal Tumor (Invading Clivus)
  - Often squamous cell CA arising from nasopharyngeal mucosal space
  - Multi-planar MR images best show invasion of clivus
Schwannoma
- T2 hyperintense, enhance
Arachnoid Cyst
- Extra-axial cyst follows CSF attenuation/signal
- Suppresses completely with FLAIR; no DWI restriction
Craniopharyngioma
- 90% Ca++, 90% cystic, 90% enhance
- May extend behind sella into posterior fossa
Neurenteric Cyst
- Round/lobulated nonenhancing, slightly hyperintense to CSF mass
- Benign malformative endodermal CNS cyst
Ecchordosis Physaliphora
- Notochord remnant
- Extends from clivus into prepontine cistern
- Hyperintense on T2WI

Image Gallery

Axial FLAIR MR reveals a hyperintense artifact

due to CSF turbulent flow. Also note sulcal hyperintensity from subarachnoid hemorrhage

Axial T1 C+ MR demonstrates luminal enhancement of a dolichoectatic basilar artery

with associated deformation of the pons

(Left) Sagittal T1WI MR shows a large mass anterior to the pons and medulla

. Note mixed hyper-, isointense signal caused by slow flow & laminated clot in this classic ASVD fusiform aneurysm. (Right) Sagittal T1 C+ MR demonstrates avid meningioma enhancement