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Neuro -- Infratentorial Masses


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What are the three questions you must answer in order to come up with a good ddx for infratentorial neoplasms?
1) Age (of pt)

2) Location (intraax vs extraax)

3) Pattern of enhancement
Of extraaxial tumors, what is another important classification?
CP angle or not
Of CP angle masses, what is another important classification?
IAC or not
In the adult, what is more common: Intraaxial or extraaxial posterior fossa tumors?
What are the 4 extraaxial lesions that you must keep in mind for adults?
Schwannoma (5th or 7th nerve)


Arachnoid cyst


Dermoid (for boards, since so characteristic)
What is the most common intraaxial infratentorial brain tumor in adults?
What is the most common primary intraaxial posterior fossa tumor in adults?
What is the ddx for intraaxial infratentorial tumor in adults?



Brainstem glioma

Cerebellar astrocytoma

How common are extraaxial posterior fossa masses in children?
Very uncommon
What extraaxial lesion presents often in posterior fossa in kids?
Vestibular schwannomas related to NF-2
What is the ddx for extraaxial infratentorial tumor in kid?
Arachnoid cyst


What are more common in kids infratentorially? Intra or extraaxial.
What are common intraaxial infratentorial tumors?
PNET (medulloblastoma)


Brainstem glioma

Cerebellar astrocytoma
Pediatric patient with well defined tumor in the pons: DDx

Thats pretty much it
What would be the only other masslike lesion of brainstem in child?
Postinfectious demyelination (ADEM), which can be tumefactive
Which intraaxial primary pediatric infratentorial tumor occurs out most laterally?
Pilocytic astrocytoma
Why is pilocytic astrocytoma out lateral?
Because its occurring in the cerebellar hemisphere
What is appearance of mural nodule in pilocytic astrocytoma?
Intensely enhancing
How do you differentiate this from cerebellar hemangioblastoma?
JUST BY AGE...hemangioblastoma is an adult tumor
Can kids get higher grade cerebellar astrocytomas?
Yes. But less common. Also see lesion laterally, but enhancement pattern totally different. Can range from low grade to GBMs.
What are the other remaining considerations for pediatric infratentorial neoplasms?

PNET (medulloblastoma)
What is true of the position of ependymoma in posterior fossa?
always MIDLINE
Because thats where the ependyma is in the posterior fossa
Where do PNET-MBs occur?
Midline as well
Do PNET-MBs have to be midline?
No, but 75% are located in the vermis.
Why are MBs and ependymomas not readily identifyable from each other in real life?
The MB infiltrates from the vermis to exophytically fill the 4th ventricle
For boards, what would differentiate ependymoma from MB?
Ependymomas are "plastic" so they ooze out of the outlet foramina like toothpaste into the extraaxial compartment
What would definitively differentiate MB from ependymoma?
If they show drop mets or subarachnoid spread, there you go.
What else would be seen in MB but not so much in ependymoma?
Brain edema, since lesion arose from brain
Where is foramen of magendie?
Posteroinferior aspect of 4th ventricle
What does Magendie connect with?
Cisterna magna
Where is are foramina of Luschka?
Paired structures at anterolateral aspect of mid to superior portion of 4th ventricle
What do they pass out into?
Low portion of the cerebellopontine angle
What is the DDx for infratentorial intraaxial masses in an adult?
1) Mets

2) Hemangioblastoma

3) Brainstem glioma

4) Cerebellar astrocytoma

5) Choroid plexus papilloma
If you see a solitary cerebellar mass in adult, what is #1 of DDx?
What about if you know its a neoplasm?
Mets, far and away
What is the most common primary infratentorial adult neoplasm?
What are the other potential adult infratentorial intraaxial masses?
Demyelinating dz

So what is the full list of adult infratent intraax masses?
1) Infarct

2) Mets

3) Infection/abscess

4) Demyelinating dz

5) Hemangioblastoma

6) Brainstem glioma

7) Cerebellar astrocytoma

8) Choroid plexus papilloma
What are common appearances of infratentorial mets?
Hyperdense mass on unenhanced CT

Ring enhancing cystic mass

Anything in between
What is one characteristic common to mets?
Lots o' edema
What do you need to do with when you see a neoplasm of the brain?

1) Intraaxial or extraaxial

2) Age of patient

3) Stick to your differential. Mets can look like anything. Don't think it looks too infiltrating to be a met. Common is common.
What does enhancement of the cerebellar folia indicate?
Subarachnoid space process
Like what?

-- Carcinomatous


--Sarcoid (?)
What appearances strongly suggest hemangioblastoma?
Cyst with enhancing mural nodule in patient of appropriate age == PATHOGNOMONIC

Solid brightly enhancing mass with prominent flow voids == VERY SUGGESTIVE
How do you classify CPA masses?
1) Intraaxial

2) Extraaxial, intradural

3) Extradural
What is the most common of these?
Extraaxial, intradural
What is the differential for extraaxial, intradural cerebellopontine angle masses?
1) Schwannoma (vestibular much more common than trigeminal)

2) Meningioma

3) Epidermoid

4) Arachnoid cyst
What are the extradural lesions in this region?
Bony tumors (chordoma, mets, myeloma)

Paraganglioma (i.e. glomus jugulare extending up from jugular foramen)
What are the intraaxial lesions in the CPA region?

Choroid plexus papilloma

How do you determine whether a lesion in the CP angle is intra or extraaxial?
If intraaxial,

1) Signal change or mass extending into pons

2) Poor demarcation from pons or cerebellum

3) Narrows CSF spaces
What if it is extraaxial?
1) Displaces or rotates pons

2) Well demarcated from pons and cerebellum

3) Widens CSF spaces
What is the most common CP angle mass?
Vestibular schwannoma
What percent of (extraaxial) CP angle tumors are schwannomas?
What proportion of those are vestibular?
Most of em
What percent of (extraaxial) CP angle masses are meningiomas?
What is the third most common CP angle mass?
What percent are epidermoid?
What is the 4th most common CP angle mass?
Arachnoid cyst
What is the first question to ask once you have established that you are dealing with an extraxial intradural mass of the CP angle?
Does it enhance
What is the DDx if it enhances?
Only schwannoma and meningioma
If it doesn't enhance, what is it?
Epidermoid or arach cyst
When you suspect an arachnoid cyst, and the lesion is slightly different in intensity than CSF on certain sequences, how must you change your differential?
Don't. It is expected to have slight differences because the CSF confined to the arachnoid cyst has different flow/behavior than free CSF in vents and cisterns, so small differences are expected.
How should arach cyst appear on FLAIR?
Nulled out
How does epidermoid look relative to CSF on different sequences?
Looks same as CSF on T1W, T1postgado, and T2.

Looks very very different from CSF on FLAIR. It does not go to signal void, but has a heterogeneous intensity distribution in an appearance of similar to cauliflower.
What is the clincher for epidermoid?
Diffusion restriction
How do you differentiate FLAIR from precontrast T1 images when the image quality is not so good?
Gray matter is hyperintense to white matter on FLAIR

White matter is hyperintense to gray matter on T1WI.
What is the main imaging differentiator between meningioma and vestibular schwannoma?
Schwannoma extends into and EXPANDS the internal auditory canal.
Where does a vestibular schwannoma originate?
Internal auditory canal, then spills out medially, where there is more room, into the CP angle cistern. So it is not really correct to say the mass is extending INTO the IAC, it is actually fleeing the IAC.
How do you describe the enlargement of the IAC caused by a vestib schwannoma?
"Funnel-shaped" enlargement
What is the appearance of the CP angle cistern portion of a vestibular schwannoma?
SPHERICAL, just like the ones Barlev showed me yesterday. Meningiomas are more broad based along a dural surface and don't look so round, with obtuse angles to the CSF space around them, while VSs form acute angle with the CP angle cistern because they are round.
What are the enhancement characteristics of vestibular schwannomas?
Variable. Can enhance uniformly or have heterogeneous contrast uptake, with regions of nonenhancement.
Why does it have heterogeneous contrast uptake and signal?
Has both Antoni A and B stroma
What is the enhancement pattern of a meningioma?
Homogeneous enhancement
What is true of 10% of CP angle meningiomas?
Enhancement extends into the IAC. But regardless, there will be NO expansion of the IAC
What sequence is used to evaluate the IAC. If you had only one sequence to choose, what would it be?
T1W post gado with fat saturation
Why fat sat?
Petrous apex and other adjacent bones may have marrow, and thus be bright signal on T1W images.
What is the best way to r/o acoustic schwannoma?
See nerve filaments going normally into the IAC
What sequence is this best accomplished with?
Do you need gado if you can see filaments all the way from pons, through CP angle cistern, through IAC into the labyrinth?
What other anomalies in the CP angle cistern region should you be aware of?
Vascular lesions: AICA aneurysms.

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