This document provides an overview of common pediatric brain tumors located in the posterior fossa (infratentorial region). It discusses the most frequently used MRI sequences for evaluating these tumors and provides clinical and imaging features of the most common tumor types, including medulloblastoma, ependymoma, pilocytic astrocytoma, and brainstem glioma. Differential diagnoses are also reviewed. Key sequences discussed are T1WI, T2WI, FLAIR, DWI, and post-contrast T1WI. Common features and imaging findings are highlighted for each tumor type in 1-3 sentences.

1. Infratentorial Pediatric Brain
Tumors
Dr. Muhammad Bin Zulfiqar
PGR IV FCPS SHL / SIMS
radiombz@gmail.com

2. AIMS
• Brief Introduction of MRI Sequences
• Posterior Fossa Tumors
• Most Common Posterior Fossa Tumors

3. Commonly Used Sequences In Brain
• T1WI
• T2WI
• FLAIR (Fluid Attenuation Inversion Recovery)
• DWI
• ADC MAP
• GRE sequence
• MR Spectroscopy

4. • T1WI
• T2WI
• FLAIR (Fluid Attenuation Inversion Recovery)
• CE T1WI

5. • DWI
• ADC MAP
• GRE sequence

6. MR spectroscopy
• MR spectroscopy (MRS) allows tissue to be interrogated for the
presence and concentration of various metabolites.
• May help in improving our ability to predict histological grade.
• lactate: resonates at 1.3 ppm
• N-acetylaspartate (NAA): resonates at 2.0
• creatine: resonates at 3.0 ppm
• choline: resonates at 3.2 ppm
ppm—Parts per million

7. MRS In Tumors

8. Posterior Fossa Tumors
Brainstem
Tectum /Cerebellum
Fourth Ventricle
CP angle
Tectum / Cerebellum
Fourth Ventricle
CP Angle
Pilocytic astrocytoma
Medulloblastoma
Hemangioblastoma
Medulloblastoma
Ependymoma
Choroid Plexus Papilloma
Acoustic neuroma
Meningioma
Choroid plexus papilloma
Epidermoid Cyst

9. Brainstem
• Glioma
• Ganglioglioma
• Astrocytoma

10. Tectum / Cerebellum
• Pilocytic astrocytoma
• Medulloblastoma
• Hemangioblastoma

11. Fourth Ventricle
• Medulloblastoma
• Ependymoma
• Choroid Plexus Papilloma

12. CP Angle
• Acoustic neuroma
• Meningioma
• Choroid plexus papilloma
• Epidermoid Cyst

13. Most Common Tumors
• Pilocytic Astrocytoma
• Medulloblastoma
• Ependymoma
• Brainstem Glioma
• NF2
• Schwanoma
• Meningioma
• Epidermoid Cyst
• Arachnoid Cyst

14. Posterior Fossa Tumors—CF
• Developemental delay
• Sleepiness
• Nausea and vomiting (area postrema)
• Truncal or gait ataxia
• Head tilt
• Cranial nerve palsies
• Macrocrania

15. Medulloblastoma
• Most common pediatric CNS malignancy(25%)
• Peak incidence 5-9 years (75% under 10years)
• M>F 3:1
• Fourth ventricle obstruction leads to hydrocephalus
• Present with headache, ataxia and vomiting
• Macrocephaly if have open sutures
• Cranial nerve abnormalities uncommon.

16. Medulloblastoma—Imaging features
• Originates from the roof of the 4th ventricle
• Superior medullary velum or inferior vermis at the level of posterior velum
medullaris
• Cleavage plane between mass and 4th ventricle
• Hyperdense to gray matter on CT (90%)
• Hypo / isointense to gray matter on T2WI
• >90 % enhance (relatively homogeneously)
• Mildly restricted diffusion.

17. CT and MRI of medulloblastoma.
• CT demonstrates a hyperdense lesion with eccentric calcifications
encompassing 4th ventricle.
• MRI demonstrates a mass centered on the roof of the fourth
ventricle. It enhances heterogeneously but vividly and has
restricted diffusion on DWI consistent with high cellularity. There
is evidence of extensive leptomeningeal spread.

18. • Proton MR spectroscopy image showing prominent choline peak in a
mass arising from roof of 4th ventricle---- Medulloblastoma.

19. CT and MRI of medulloblastoma.

20. Classical Medulloblastoma—
Leptomeningeal spread

21. Medulloblastoma with anaplastic features

22. Medulloblastoma with extensive
nodularity

23. DDX for Medulloblastoma
• Ependymoma is usually more heterogeneous with calcifications and
hemorrhage, extends out of the 4th ventricle.
• Juvenile pilocytic astrocytoma is usually hemispheric with cystic and
nodular component.
• Brainstem glioma location is main differentiating point.
• Atypical rhabdoid tumor can be indistinguishable by imaging but are
usually younger (< 3 years old), often more heterogeneous.

24. Infratentorial Ependymoma
• Clinical features
• Third most common posterior fossa tumor after medulloblastoma
and brainstem glioma (15% Infratentorial tumors in kids)
• Usually older than medulloblastoma
• Peak incidence 1-5 years, but may present as adolescent
• More subacute and prolonged presentation than medulloblastoma
• Younger age associated with worse prognosis.

25. Infratentorial Ependymoma
• Plastic tumor arising from floor of the 4th ventricle
• Extends through foramen of luschka and magendie
• Calcifications (50%)
• Cysts and hemorrhage common
• Heterogeneous enhancement
• No restricted diffusion

26. Infratentorial Ependymoma
• MRI demonstrates a large heterogeneous mass centred in then fourth ventricle
and herniating out of the left foramen of Luschka and magendi. It is mostly
isointense to the adjacent cerebellum on T1 weighted images, hyperintense on T2
(with areas of higher signal / cystic change), and without significant restricted
diffusion. Post contrast images demonstrate vivid but heterogeneous contrast
enhancement.
• There is no surrounding oedema, and although the fourth ventricle is effaced
there are no features to suggest significant hydrocephalus on the images
provided.

27. Infratentorial Ependymoma

28. Infratentorial Ependymoma

29. Cerebellar pilocytic astrocytoma
• Middle aged child 5-15 years old
• > 80 % under 20
• Prolonged duration of symptoms—years
• Headache, nausea, vomiting
• Visual loss (optic pathway)
• Ataxia / cerebellar signs.

30. Cerebellar pilocytic astrocytoma
• Location
• Cerebellum (60%) > optic nerve chiasma (25-30%) > near 3rd ventricle >
brainstem
• Cyst and enhancing mural nodule
• > 95 % enhance
• 50 % cyst and enhancing mural nodule
• 40 % solid lesion with heterogeneous enhancement and necrotic center
• 10 % solid homogeneous enhancement
• Cyst contents usually bright on T2
• Leptomeningeal mets rare.

31. Cerebellar pilocytic astrocytoma
• MRI shows heterogeneous cystic mass with mural nodule
hypointense on T1WI, Hperintense on T2WI with surrounding edema
and mural nodule shows avid enhancement on Postcontrast images.

32. Cerebellar pilocytic astrocytoma

33. Cerebellar pilocytic astrocytoma

34. Brainstem gliomas
• 15 % of all brain tumors.
• 25-30 % of all brain tumors in chidren.
• Most are low grade astrocytoma
• Predominantly in the pons
• Less frequently in the medulla
• May infiltrate extensively throughout the brainstem.

35. Brainstem glioma—clinical
• Subtyped by location
• Location
• Tectal
• Mesencephalic
• Pontine (most common location)
• Medullary
• Tectal gliomas and NF associated tumors are distinct entities.

36. Brainstem glioma—clinical
• Peak incidence 3-10 years
• M=F
• Most have poor prognosis
• JPA, NF1 associated and tectal glioma have good prognosis
• Median survival 1 year, 20 % survival at 2 years
• Tx—radiation therapy only.

37. Brainstem glioma—Imaging features
• T2 hyperintense, T1 low to isointense
• Enhancement variable, associated with higher grade tumors and
worse prognosis
• DWI usually not restricted, worse grade and prognosis if positive.
• May encase basilar artery, without narrowing it
• Beware the effects of steroid therapy on enhancement characteristic.

38. Brainstem glioma
• MRI shows expansile pons
based T2 hyperintense mass
with minimal enhancement on
Postcontrast images.

39. Brainstem glioma

40. DDX Brainstem glioma
• Brainstem / rhombencephalitis—usually febrile
• Listeria, herpes, West nile, EBV, etc
• NFI, characteristic increased T2 signal in brainstem, cerebellum, basal
ganglia
• Increase in childhood, decrease in adolescence
• Histiocytosis
• Langehans cell and hemophagocytic lymphohistiocytosis
• Look for other sites of disease in brain, meninges, pituitary and skull.
• Acute disseminated encephalomyelitis (ADEM)

41. DDX of Posterior Fossa Tumors
Tumor Age
(yrs)
Site Commonest
feature
TIWI T2WI Post Contrast Diffusion
Restriction
Medulloblastoma 5-9 Roof of 4th ventricle Fast growing, ho
fits, cerebellar
ataxia
Hypo Variable Heterogeneous,
Leptomeningeal
positive
Pilocytic
Astrocytoma
5-15 hemispheric Cyst with mural
nodule
Surrounding
edema
Hypo Hyper Mural nodule
enhances
No
Brain Stem Glioma 3-10 Pons, midbrain
medulla
Slow growing Hypo Hyper Minimal
enhancement
usually
No
Ependymoma 1-5 Floor of 4th ventricle Slow growing
Foramina Ext.
Iso / hypo Hyper with cystic
components
Vivid
heterogeneous
No

42. Cerebellopontine angle masses
• Acoustic neuroma (60-92%)
• Meningioma
• Choroid plexus papilloma
• Epidermoid Cyst

43. Acoustic Neuroma
• Comprises 60-92 % of CPA lesions.
• Usually unilateral
• Arise from schwann cells, commonly within IAC.
• Occur with equal frequency on the superior and inferior vestibular
nerves.
• Greatest density of S. cells at scarpa ganglion.
• Majority of cases are sporadic.
• Rarely on Cochlear division of the 8th CN.

44. Acoustic Neuroma
• NF2:
• Genetic defect on long arm of chromosome 22.
• Autosomal dominant.
• Bilateral or early in life.
• Associated with intracranial meningiomas and spinal cord tumors.
• Tumors suppressor gene is absent.

45. Features
• Centered on porous acousticus
• Acute angles to petrous bone
• Often involves the IAC
• Homogeneous enhancement
• No dural tail
• No calcifications

46. Acoustic Neuroma MRI Findings.

47. Meningioma
• Second most common CPA lesion 3-7 %
• Arise from cap cells near arachnoid villi which are more prominent
near cranial nerve foramina and venous sinuses.
• Usually arise from posterior surface of the petrous bone and usually
do not extend to external auditory canal
• Ataxia, Nystagmus, facial hyperesthesia

48. Meningioma
• Tumors generally hemispherical with obtuse angles to petrous bone
• Dural tail often present (50-75%)
• May herniate into middle fossa (50%)
• May show calcification (25%)
• Pial blood vessels with flow voids may be present at the margins

49. Meningioma

50. Acoustic Neuroma Vs Meningioma

51. DD of Acoustic Neuroma / Meningioma
Features Acoustic Neuroma Meningioma
Site of origin Schwan cells, Internal
auditory canal
Posterior surface petrous bone
Angle with petrous bone Acute Obtuse
Internal auditory canal extension Yes No
Dural tail No Yes
Enhancement Intense Intense---Cogwheel / Spiral pattern
Extension to Middle cranial fossa No Yes (50% cases)
Hyperostosis No Yes

52. Epidermoid
 Irregularly lobulated
 Insinuating
 Common in CP angle, 4th ventricle, supra & parasellar regions.
 Intra cerebral < 10%
 Incorporation during 4-5th week of development.
 No dermal appendages & hair follicles.
 Dermoids contain dermal appendages.

53. Epidermoid
 Similar to CSF on T1 & T2
 High signal in case of white epidermoids
 Incomplete nulling on FLAIR
 DWI – Restriction
 No enhancement, Thin enhancement at the periphery
 25% may show rim enhancement
 White epidermoid – More protein and debris ---- high signal on
T1 & CT.

54. Epidermoid
• Characteristic appearance of an epidermoid cyst in
the cerebellopontine angle. The lesion has similar signal intensity to
CSF on both T1 and T2 weighted sequences, but is 'dirty' on FLAIR.
The diagnosis is confirmed on DWI which demonstrates very bright
signal.

55. Arachnoid Cyst
• Benign intra-arachnoid CSF filled mass
• Almost always extraaxial
• No restricted diffusion

56. • MRI show TI, Flair hypo, T2 hyper non enhancing lesion at right
cerebellopontine angle (following CSF signals on all sequences) with
no diffusion restriction.

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