Medulloblastoma

KEY FACTS

TERMINOLOGY

• Medulloblastoma (MB)

○ Malignant, invasive, highly cellular embryonal tumor

• 4 distinct MB molecular subgroups

○ Wingless (WNT)

○ Sonic hedgehog (SHH)

○ Group 3

○ Group 4

IMAGING

• MB subgroups arise in different locations

○ Midline (4th ventricle)

– Predominately (but not exclusively) groups 3 and 4

○ Cerebellar peduncle/CPA cistern: WNT

○ Cerebellar hemispheres (lateral): SHH

• NECT: 90% hyperdense (Ca++, hemorrhage uncommon)

• MR: > 90% enhance (group 4 minimal/no enhancement)

○ Restricted diffusion, low ADC

○ T1 C+ essential to detect CSF dissemination

TOP DIFFERENTIAL DIAGNOSES

• Atypical teratoid/rhabdoid tumor

• Ependymoma

• Cerebellar pilocytic astrocytoma

PATHOLOGY

• WHO grade IV (overstates risk of WNT MB)

• Microscopy

○ "Small round blue cell" tumor

○ Neuroblastic (Homer Wright) rosettes (40%)

• Most common histologic type = classic MB

• Less common subtypes: Desmoplastic, MB with extensive

nodularity, large cell/anaplastic

CLINICAL ISSUES

• Most common malignant pediatric brain tumor

• Prognosis varies with subgroup, mutations

○ WNT best, group 3 worst

(Left) Axial graphic shows a

spherical tumor centered in

the 4th ventricle ﬊, typical of

classic medulloblastoma (MB).

Cerebrospinal fluid (CSF)

dissemination ("sugar icing") is

common at initial diagnosis

and is shown in blue ﬈.

(Right) Axial T1WI shows

imaging findings of classic MB,

with a rounded, slightly

hypointense mass ﬈ in the

4th ventricle.

(Left) Axial T1 C+ FS in the

same case shows the mass

mass ﬇ enhances strongly,

somewhat heterogeneously.

No CSF spread is identified.

(Right) DWI in the same case

shows strongly restricted

diffusivity ﬇. Surgery

disclosed MB, desmoplastic

subtype. The midline 4th

ventricle is the 2nd most

common location for

desmoplastic MBs and the

most common site for classic

MBs. Histopathology disclosed

desmoplastic MB, Sonic

hedgehog subtype.

Cysts, and Disorders

Brain: Pathology-Based Diagnoses: Neoplasms,

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