High-Yield Anatomy

The PF extends from the tentorium cerebelli to the foramen magnum. It houses the brainstem, cerebellum and neurovascular structures.

Fig1 depicts IV ventricle anatomy. The choroid plexus is located along its roof and protrudes into the cerebellopontine angle (CPA) through foramina of Luschka.

Osseous boundaries include the clivus anteriorly, petrous and mastoid temporal bones laterally and occipital bone posteroinferiorly.

Skull base houses the:

• Jugular foramen
• pars nervosa: CN IX, inferior petrosal sinus
• pars vascularis: CN X, XI, jugular bulb, posterior meningeal artery
• Hypoglossal canal: CN XII
• Internal acoustic canal (IAC): CN VII, VIII, AICA loops

Diagnostic procedure and imaging considerations:

Initial approach should seek location of origin:

• Intra-axial (cerebellar or brainstem)
• Intraventricular
• Extra-axial (CPA cistern)
• Skull base

Exclude context-appropriate tumor mimics, like tumefactive demyelination, vascular or infectious diseases – Fig2.

Heavily T2-weighted MR imaging can precise location and tumor extent, especially when contrast is contraindicated.

Assessment of CSF seeding with neuraxis imaging and lumbar puncture, brainstem compression, hydrocephalus, herniation and cancer predisposition syndromes is recommended.

Cerebellar masses:

The most common aPF “mass” is subacute stroke. The number-one neoplasm is cerebellar metastasis. The most common primary tumor is hemangioblastoma.

1. Metastasis – Fig3

Usually display aggressive features like hemorrhagic necrosis, extensive edema, invasion of neighboring structures (eg, meninges, venous sinuses, bone), rapid growth and/or multiplicity.

Key primary tumors:

• Lung
• Breast
• Melanoma
• Renal cell carcinoma (RCC)
• Thyroid

2. Hemangioblastoma (WHO grade 1, mesenchymal tumor) – Fig4

Commonly presents as a well-circumscribed, cyst-with-mural-nodule or solid mass (60% vs. 40%) with minimal edema, like pilocytic astrocytoma (PCA). Some distinguishing features include:

• Middle-age presentation (30-60yo) in sporadic forms
• Prominent vessels
• Vividly-enhancing nodule
• Location near pial surface
• Non-enhancing cyst walls
• Absent calcifications
• Multiplicity – in von Hippel-Lindau (VHL) disease – Fig5
• Arteriovenous shunting on angiography with dense capillary blush (vs.AVMs)

In VHL, lookout for retinal hemangioblastomas, endolymphatic sac tumor (ELST), choroid plexus papilloma and RCC.

3. PCA (WHO grade 1, circumscribed glioma) – Fig6

Typically arises from midline structures in individuals<40yo.

Unlike pediatric cerebellar-type, adult PCA is rare, typically supratentorial, shows variable enhancement and cyst formation, higher recurrency and only 20% association with BRAF-KIAA1549 fusion gene.

Adult PCAs are commonly solid and heterogenous, often mimicking high-grade gliomas or metastasis, including perfusion and spectroscopy findings. Higher ADC values support PCA.

Enhancement may be absent or faint, oftentimes described as “cotton-wool” or “out-of-focus”.

Cerebellar PCA is not Neurofibromatosis type 1(NF1)-associated, as are optic pathway gliomas.

High-grade astrocytoma with piloid features (not yet graded), now a separate entity, arises de novo or from pre-existing PCA, with preferential involvement of cerebellum in middle-aged adults or midline thalami, especially in NF1. Diagnosis is made by DNA-methylation as heterogenous imaging and histology resemble glioblastoma.

4. Medulloblastoma (WHO grade 4, embryonal tumor) – Fig7

Most common primary cerebellar tumor in individuals<20yo. One-quarter affects 20-40 year-olds.

Unlike paediatric-type, adult medulloblastomas show predominant:

• Lateral hemispheric location
• SHH-activated TP53-wildtype molecular group
• Desmoplastic/nodular/medulloblastoma with extensive nodularity (MBEN) histology
• Larger cysts
• Poorer enhancement
• Better prognosis

Characteristically circumscribed and hypercellular, originating CT hyperdensity, T2 hypointensity and ADC values lower than parenchyma.

When vermian (25%), intraventricular protrusion occurs from the roof (vs.ependymoma).

Peripheral location may mimic meningioma, in which spectroscopy may demonstrate alanine peak.

Both hemangioblastomas and medulloblastomas can occasionally arise extra-axially.

Associations:

SHH-activated:

• Gorlin-Goltz Syndrome: look for odontogenic keratocysts and dural calcifications.
• Li-Fraumeni Syndrome
• ELP1-MB Syndrome
• Fanconi anemia

WNT-activated:

• Adenomatous polyposis
• Turcot syndrome

5. Lhermitte-Duclos disease (WHO grade 1, glioneuronal tumor) – Fig8

• 20-40yo adults
• Non-enhancing thickened cerebellar folia (+left hemisphere)
• Preserved cortical striations
• Prominent venous channels (which may enhance, show elevated perfusion or flow-voids)
• Decreased choline on spectroscopy (vs.high-grade tumors)

Differential diagnoses: MBEN(SHH-activated), cerebellitis, subacute infarction.

In adults, PTEN mutation is invariably found, in association with Cowden/COLD Syndrome. Lookout for Cowden-associated neoplasms (breast, endometrial, thyroid).

6. Others

Although rare, infiltrative aPF-masses displaying marked hyperperfusion or hypercellularity suggest glioblastoma – Fig9 – or lymphoma – Fig10, respectively.    

Diffuse/infiltrative gliomas grades 2-4 can affect the cerebellum but in the PF, they account for most brainstem gliomas, particularly in the young.  

Similarly, PF ganglioglioma (WHO grade 1-3, glioneuronal turmor) mainly affects the dorsal medulla oblongata, expanding nucleus cuneatus/gracilis. Cerebellar ganglioglioma often originates in the dentate nucleus with an accompanying FLAIR rim.

Brainstem tumors:

Conventionally, tumors (eg, focal tectal or thalamo-peduncular glioma) portend a lower grade than pontine or bulbar neoplasms, wherein dorsal exophytic location is favorable (PCA, ganglioglioma) – Fig11 – unlike diffuse or ventral involvement – Fig12.

NF1-associated brainstem tumors occur mostly in the lower medulla and tend to act more indolently than non-NF1.

Most pontine gliomas are H3 K27M-altered (WHO grade 4, pediatric-type diffuse glioma), typically:

• 30-50% of adult brainstem gliomas (<40yo)
• Midline (pons>medulla>midbrain)
• Thalamic and spinal cord involvement primarily affects young adults
• Expansion >50% of brainstem diameter
• Poorly-marginated T2-hyperintensity
• Little-to-no enhancement
• Basilar artery engulfment
• “Flat floor of IV ventricle sign”
• Choline peak on spectroscopy
• Possible exophytic growth along cranial nerves

IV ventricle tumors:

Arise off ventricular wall (ependymoma, subependymoma, medulloblastoma, rosette-forming glioneuronal tumor [RGNT]) or lumen (choroid plexus papilloma/carcinoma, meningioma, metastasis).

1. Ependymoma (WHO grade 2-3, ependymal tumor) – Fig13.

Although ependymomas in adults occur mostly in the spinal cord and supratentorium, the primary PF intraventricular tumor is subgroup PF-B ependymoma.

Compared to laterally-located pediatric PF-A type, they show:

• Predilection for<40yo adults.
• Midline floor origin, near Magendie/obex
• “Soap-bubble” sign
• Better prognosis

Ependymomas generally show:

• Plasticity through foramina
• Heterogeneity (calcifications, cysts, hemorrhage)
• Avidly-enhancingsolid component
• CT isodensity
• Intermediate ADC
• Possible transependymal invasion
• CSF dissemination
• Hydrocephalus

2. Subependymoma (WHO grade 1, ependymal tumor) – Fig14

Originates in:

• >40yo
• Midline floor, near obex
• Purely intraventricular
• T2/FLAIR-hyperintense non-enhancing mass
• No hydrocephalus or parenchymal edema

3. Choroid plexus papilloma (WHO grade 1) – Fig15

• Most near obex in adults (vs.ventricular trigone in children)
• Well-defined, lobulated solid mass
• Vivid frond-like vascular enhancement
• Hydrocephalus

Suspect malignancy when:

• Larger
• Heterogeneously-enhancing
• Hemorrhagic
• Extensive edema
• Choroid plexus carcinoma (WHO grade 3) occurs mostly in infants. Consider metastasis!

Associations: Hemangioblastoma, Aicardi Syndrome, VHL.

4. RGNT (WHO grade 1, glioneuronal tumor)

Consider when:

• 20–40yo
• Midline-located: superior IV ventricle or vermis
• Multicystic
• Patchy/bell-pepper enhancement

 CPA masses:

The most common extra-axial aPF-mass is vestibular schwannoma, followed by meningioma and epidermoid cyst.

Enhancing masses:

1. Schwannoma (WHO grade 1, nerve sheath tumor) – Fig16

• Mixed solid-cystic
• Isodense on CT
• High T2/ADC values
• Avidly-enhancing (vs.neurofibroma)
• Cystic-hemorrhagic degeneration
• No calcifications
• Acute angle with skull base
• Porus acoustics widening
• Origin near IAC fundus
• “Ice-cream” or “dumbbell”-shaped
• Hydrocephalus
• Mass effect on cerebellar-pontine junction

Parenchymal edema elicits alternative diagnoses (metastasis, lymphoma).

Bilateral IAC involvement suggests NF2, along with multiple ependymomas and meningiomas in the young or leptomeningeal disease (carcinomatosis, leukemia, sarcoid, infection) – Fig17.

2. Meningiomas (WHO grade 1-3) – Fig18

• Broad-based
• Obtuse angle with skull base
• Possible extension into IAC without expansion
• Dural tail
• Arterial narrowing
• Hyperperfusion
• Calcifications
• Hyperostosis

Bone erosion suggests metastasis, plasmocytoma, hemangiopericytoma, paraganglioma or ELST.

CSF-density masses:

1. Epidermoid cyst – Fig19

• Non-enhancing
• ADC values resemble parenchyma’s

2. Arachnoid cyst – Fig20

• CSF signal on all sequences
• May rarely hemorrhage

3. Neuroenteric cyst – Fig21

• “Floating lesion”
• Ponto-medullary junction
• No restriction diffusion
• No FLAIR suppression

T1-hyperintensy elicits the following diagnoses:

• Thrombosed aneurysm – Fig22
• Hemorrhagic or melanotic schwannoma
• Cholesterol granuloma – Fig23
• White epidermoid
• Ruptured dermoid
• Lipoma
• Neuroenteric cyst
• Paraganglioma (“salt and pepper” – Fig24)
• ESLT
• Cerebellar liponeurocytoma (WHO grade II, glioneuronal tumor)
• Melanocytic neoplasm

Skull base tumors:

When evaluating lytic lesions entertain the possibility of metastases – Fig25 – or plasmocytoma/multiple myeloma – Fig26 – especially if associated mass, multiplicity, additional calvarial involvement or “punched-out” morphology in the latter.

In jugular foramen primary lesions, bone CT predicts histology by demonstrating:

• Smooth remodeling in schwannomas
• Permeative sclerosis in meningiomas
• Permeative erosion in paragangliomas

Exclude contiguous spread from ELST (moth-eaten erosion of vestibular aqueduct with intratumoral spicules) or H&N carcinomas (frank lytic destruction) – Fig27.

Intraosseous venous malformations (IOVM) demonstrate expansile honeycomb appearance with intralesional bone spicules. Most occur at the facial nerve geniculate ganglion followed by IAC, but can occur anywhere and be mistaken for paragangliomas given their avid enhancement.

Paragangliomas’ distinguishing features include:

• Rapid contrast wash-in and wash-out (vs.Schwannomas, IOVM)
• Arteriovenous shunting (vs.Chordomas/chondrosarcomas)
• Intense tumor blush
• Enlarged feeding vessel (usually ascending pharyngeal artery)

Genetic syndromes associated with multiple paragangliomas:

• Familial paraganglioma-phaeochromocytoma syndrome (SDH gene)
• Multiple endocrine neoplasia syndrome (RET oncogene)
• VHL
• NF1

Clival tumors:

During normal aging the clivus transitions from hematopoietic to fatty marrow. T1 hypointensity in older adults is suspicious of neoplasm or diffuse systemic disease.

1. Chordoma:

• Notochord-derived malignancy in adults, mostly occurring in sacrum
• Most common clival primary neoplasm
• Midline
• Bone destruction/sequestra
• No calcification
• Extraosseous extension
• T2-hyperintense, enhancing soft-tissue mass
• Pontine indentation (“thumb sign”)

2. Benign notochordal cell tumor

• Benign notochord-derived neoplasm
• Purely intraosseous
• Mild osteosclerosis
• Little-to-no enhancement
• Long-term stability

3. Ecchordosis physaliphora – Fig28

• Benign ectopic notochordal remnant
• Midline
• Prepontine intradural location
• T2-hyperintense soft-tissue mass
• Non-enhancing
• Stalk-like or corticated clival defect
• Long-term stability

4. Chondrosarcoma:

• Cartilage-derived malignancy
• Off-midline (petro-occipital fissure)
• Rings-and-arcs calcifications (chondroid matrix)
• ADC values>>Chordoma
• Bone destruction