Medulloblastoma

Slide 1 · Medulloblastoma

Malignant embryonal tumor of the cerebellum | Morphology, molecular groups, dissemination and prognosis

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Define medulloblastoma and explain its anatomic restriction.

Recognize the morphologic patterns and immunophenotype.

Relate molecular groups to location and clinical behavior.

Explain CSF dissemination and the significance of risk heterogeneity.

Slide 3 · Medulloblastoma is an embryonal CNS tumor

Embryonal neoplasms have primitive or relatively undifferentiated cytology.

Limited neuronal differentiation may be present; Glial differentiation is less common.

Medulloblastoma is the most common embryonal CNS tumor.

Embryonal phenotype

Primitive-appearing cells

Variable differentiation

Slide 4 · Its pediatric importance is substantial

Approximately 20% of pediatric brain tumors to medulloblastoma.

This proportion describes the pediatric brain-tumor case mix, not all childhood cancers.

~20%

Of pediatric brain tumors (Robbins, 11th ed.)

Slide 5 · The primary site is the cerebellum

Medulloblastoma is cerebellar by definition.

A similar primitive tumor elsewhere in the CNS should not automatically receive this designation.

Anatomic site and morphology must be integrated.

Cerebellum

Fourth ventricle

Other CNS sites

Named regions only; Position is diagrammatic

Slide 6 · All medulloblastomas are CNS WHO grade 4

The grade reflects the intrinsically malignant biologic category.

Histologic variants and molecular groups remain clinically heterogeneous.

Grade 4 alone does not resolve the major prognostic differences.

One grade: CNS WHO 4

Different morphologic patterns

Different molecular risks

Slide 7 · Rapid growth creates local mechanical danger

Expanding tumor can obstruct cerebrospinal fluid flow.

The consequence is hydrocephalus, independent of distant dissemination.

Local mass effect and biologic subtype are different dimensions of disease.

Cerebellar tumor growth

CSF-flow obstruction

Hydrocephalus

Slide 8 · Why the fourth ventricle matters

Medulloblastoma arises in the posterior fossa near ventricular CSF pathways.

Obstruction of CSF outflow provides the mechanism for hydrocephalus.

This is a location-dependent consequence, not a unique histologic marker.

Posterior fossa tumor

Ventricular CSF pathway

Impaired CSF passage

Slide 9 · Two complementary dimensions of disease

Local expansion explains CSF obstruction and hydrocephalus.

Leptomeningeal dissemination explains deposits away from the cerebellum.

Both phenomena can arise from the same primary tumor.

LOCAL

CSF obstruction

Hydrocephalus

DISSEMINATED

Subarachnoid spread

Spinal deposits

Slide 10 · Classification requires more than one axis

Molecular classification examines characteristic genetic alterations.

Histologic classification examines cellular and architectural patterns.

Clinical risk depends strongly on tumor subtype.

Molecular group

Histologic pattern

Clinical behavior

Slide 11 · At least four molecular groups are recognized

WNT-activated, SHH-activated, group 3 and group 4 are the principal categories described.

The names refer to biologic classification rather than four gross appearances.

WNT-activated

SHH-activated

Group 3

Group 4

Slide 12 · WNT-activated tumors involve a developmental pathway

The WNT/β-catenin pathway participates in embryogenesis and tissue homeostasis.

Pathway alteration identifies one major medulloblastoma group.

WNT activation has strong prognostic relevance in the cited classification.

WNT/β-catenin signaling

Pathway alteration

WNT-activated tumor

Slide 13 · SHH relates to cerebellar granule-cell proliferation

Sonic hedgehog signaling participates in proliferation of cerebellar granule cells.

Aberrant activity is implicated in the SHH-activated molecular group.

This provides a developmental link between pathway biology and tumorigenesis.

SHH signaling

Granule-cell proliferation

SHH-activated group

Slide 14 · Groups 3 and 4 are separate molecular categories

Groups 3 and 4 alongside WNT and SHH tumors.

They should not be relabeled as SHH-negative variants of a single entity.

Their prognoses cannot be inferred from morphology alone.

GROUP 3

Distinct molecular group

Adverse overall prognosis

GROUP 4

Distinct molecular group

Do not infer risk from name alone

Slide 15 · Molecular grouping is not a histologic stain

A small-cell appearance does not itself establish WNT, SHH, group 3 or group 4.

A molecular classification requires molecular evidence.

Morphologic correlations provide clues, not complete molecular identity.

Microscopic phenotype

Molecular assessment

Integrated designation

Slide 16 · SHH activation does not imply uniform risk

Robbins specifically highlights SHH-activated/TP53-mutant disease among the poorest-prognosis categories.

The TP53 qualifier changes the interpretation of an otherwise broad SHH label.

SHH-activated

Molecular family

Heterogeneous outcome

SHH + TP53-mutant

Specified high-risk subset

Poorer prognosis

Slide 17 · Group 3 is a poor-prognosis category

Group 3 is identified among the categories with the worst outcomes.

This is a group-level observation rather than a guarantee of outcome in an individual case.

Molecular subtype modifies prognosis

Group 3 is singled out as an unfavorable category.

Slide 18 · WNT-activated tumors have the best overall prognosis

The WNT group as the most favorable of the major categories.

Overall group prognosis should not be conflated with a patient-specific prediction.

WNT-activated

Most favorable overall prognostic group.

Slide 19 · Anatomic preferences differ between groups

Molecular group

Location favored

WNT-activated

Anterior fourth ventricle / middle cerebellar peduncle

SHH-activated

Vermis / lateral cerebellum

WNT-activated tumors are often near the anterior fourth ventricle or middle cerebellar peduncle.

Slide 20 · Location is supportive, not definitive

A tumor near the fourth ventricle is not automatically WNT-activated.

Vermian or lateral cerebellar position does not alone prove SHH activation.

Integrated interpretation avoids a location-only molecular diagnosis.

Observed location

Probabilistic clue

Require molecular correlation

Slide 21 · Grossly, the tumor is gray and friable

The lesion is often well circumscribed macroscopically.

Gray color and friability are characteristic reported appearances.

Circumscription does not make a WHO grade 4 tumor benign.

Well circumscribed ≠ biologically indolent

Gray, friable gross tumor; Malignant embryonal behavior.

Slide 22 · Relationship to cerebellar folia has consequences

The tumor may extend toward the surfaces of cerebellar folia.

The leptomeninges may become involved.

Surface extension offers a path toward meningeal/CSF dissemination.

Cerebellar primary

Folia / surface extension

Leptomeningeal involvement

Slide 23 · Macroscopic boundaries do not define extent

A grossly delineated cerebellar lesion may coexist with leptomeningeal spread.

The primary mass and disseminated component must be conceptually distinguished.

PRIMARY MASS

Gray / friable

Often circumscribed

DISEASE EXTENT

Meningeal spread

May be more extensive

Slide 24 · The dominant microscopic pattern is high cellularity

Schematic morphology — not a histologic specimen

Dense sheets of primitive-appearing cells typify medulloblastoma.

Little intervening cytoplasm gives a crowded blue-cell impression.

Architectural patterns allow further histologic subclassification.

Slide 25 · Small primitive cells have high nuclear-to-cytoplasmic ratios

Schematic morphology — not a histologic specimen

The cytoplasm is scant relative to the nucleus.

Nuclei are often hyperchromatic.

Cells may be elongated or crescent shaped.

Slide 26 · Nuclear shape is variable within the primitive-cell spectrum

Schematic morphology — not a histologic specimen

Crescent-shaped and elongated nuclei are described alongside small dark nuclei.

No single nuclear silhouette is sufficient to establish the diagnosis.

Slide 27 · Frequent mitoses support aggressive proliferation

Schematic morphology — not a histologic specimen

Mitoses are abundant in the classic small-cell sheets.

This finding is compatible with the high-grade biology of the tumor.

Mitotic activity is one component of morphology, not the molecular grouping.

Slide 28 · Ki-67 indexes the proliferative compartment

A high proportion of Ki-67-positive tumor cells.

Ki-67 is a proliferation marker, not proof of neuronal differentiation.

The cited passage does not supply a universal diagnostic cutoff.

Tumor-cell nuclei

High Ki-67 labeling

Proliferative activity

Slide 29 · Synaptophysin supports neuronal differentiation

Classic medulloblastoma often expresses synaptophysin.

The finding is consistent with partial neuronal differentiation.

Immunoreactivity must be interpreted with morphology and anatomic context.

Primitive cells

Neuronal differentiation

Synaptophysin expression

Slide 30 · GFAP expression is less common

Glial markers such as GFAP are expressed less often than neuronal markers.

The expected immunophenotype is not an absolute single-stain rule.

NEURONAL

Synaptophysin

Often expressed

GLIAL

GFAP

Less common expression

Slide 31 · Interpret the stains by the question they answer

Assay / stain

Primary interpretive role

H&E

Cytology, density, architectural pattern

Ki-67

Proliferation assessment

Synaptophysin

Neuronal differentiation

GFAP

Glial differentiation (less common)

H&E shows cell morphology and architecture.

Slide 32 · Homer Wright rosettes contain central neuropil

Schematic morphology — not a histologic specimen

Classic medulloblastoma can form Homer Wright rosettes.

Tumor cells are arranged around a central neuropil-rich zone.

The center does not represent an epithelial or ependymal lumen.

Slide 33 · Recognize the rosette construction

Schematic morphology — not a histologic specimen

Look for a central fibrillary neuropil rather than an empty true lumen.

Surrounding primitive cells form the recognizable rosette pattern.

The structure expresses neuronal differentiation in a primitive tumor.

Slide 34 · Homer Wright rosettes are not unique to medulloblastoma

Robbins also notes Homer Wright rosettes in neuroblastoma.

Therefore the rosette is a morphological pattern, not a site-independent diagnosis.

Cerebellar location and other findings remain decisive context.

MEDULLOBLASTOMA

Cerebellar primary

Can show rosettes

NEUROBLASTOMA

Different primary context

Can show rosettes

Slide 35 · The classic histologic pattern combines several findings

Sheets of small primitive cells and high cellularity form the background.

Homer Wright rosettes may be evident.

Synaptophysin expression supports neuronal differentiation.

Primitive-cell sheets

Homer Wright rosettes

Neuronal immunophenotype

Slide 36 · Do not reverse a histologic–molecular association

The textbook associates classic morphology with WNT-activated tumors.

That association does not establish WNT activation in an individual classic-appearing lesion.

Molecular and histologic categories should be recorded separately.

Classic histology

Possible molecular association

Molecular confirmation

Slide 37 · Desmoplastic/nodular architecture is biphasic

Schematic morphology — not a histologic specimen

Reticulin-rich internodular tissue contrasts with pale, more differentiated nodules.

The two compartments differ in stromal response and cellular appearance.

Slide 38 · Internodular zones contain a desmoplastic response

Schematic morphology — not a histologic specimen

Collagen and reticulin deposition mark the internodular areas.

The stromal pattern is a defining diagnostic clue.

It is the contrast with the nodules that creates the characteristic architecture.

Slide 39 · Pale nodules represent partial neuronal maturation

Schematic morphology — not a histologic specimen

Pale islands contain more neuropil and show more advanced neuronal differentiation.

The adjective “pale” reflects their appearance relative to the surrounding cellular tissue.

Slide 40 · Reticulin distribution explains the nodular pattern

Schematic morphology — not a histologic specimen

Reticulin-rich internodular tissue is separated from more differentiated pale nodules.

A reticulin stain can demonstrate the tissue-compartment distinction.

Slide 41 · Desmoplastic/nodular tumors usually map to SHH

Most desmoplastic/nodular cases with the SHH-activated group.

This is a useful morphomolecular clue but molecular characterization retains independent value.

Desmoplastic/nodular morphology

Strong SHH association

Integrated molecular designation

Slide 42 · Large cell/anaplastic morphology changes nuclear appearance

Schematic morphology — not a histologic specimen

Large and irregular vesicular nuclei contrast with the classic small hyperchromatic cells.

Prominent nucleoli are characteristic of this histologic pattern.

Slide 43 · Cell wrapping creates a cell-in-cell appearance

Schematic morphology — not a histologic specimen

Tumor cells may appear to wrap around neighboring cells or dying-cell nuclei.

Cell wrapping is among the described large cell/anaplastic features.

Slide 44 · High turnover accompanies anaplastic morphology

Mitotic and apoptotic indices are high.

The pattern combines proliferative and cell-death features.

The diagnosis is based on the aggregate histomorphology, not one feature.

Large / atypical nuclei

High mitotic index

High apoptotic index

Slide 45 · Compare the principal histologic patterns

Pattern

Defining discriminators

Classic

Primitive-cell sheets; Homer Wright rosettes

Desmoplastic / nodular

Pale neuropil-rich islands; Reticulin-rich stroma

Large cell / anaplastic

Vesicular nuclei, prominent nucleoli, wrapping, high turnover

Classic: Sheets of primitive cells and Homer Wright rosettes.

Slide 46 · Histology and molecular subtype answer different questions

Histology describes what the cells and tissue look like.

Molecular grouping classifies underlying oncogenic biology.

Risk interpretation benefits from both rather than substituting one for the other.

HISTOLOGY

Architecture / cytology

Observed phenotype

MOLECULAR

WNT / SHH / 3 / 4

Biologic subtype

Slide 47 · The subarachnoid space is a dissemination route

Medulloblastomas tend to spread into the subarachnoid compartment.

Tumor cells can disseminate within CSF rather than only expanding contiguously.

Cerebellar primary

Subarachnoid access

CSF dissemination

Slide 48 · Dissemination may coat CNS surfaces

Extensive surface growth is likened to “icing”.

This is a descriptive growth pattern, not a separate molecular group.

CSF-disseminated cells

Surface implantation

Extensive meningeal coating

Slide 49 · Drop metastases can involve the cauda equina

Disseminated tumor may form nodular deposits along the neuraxis.

The cauda equina as a favored location for drop metastases.

Intracranial primary

CSF dissemination

Cauda equina deposits

Slide 50 · Primary tumor and spinal deposits share a route

Cerebellar primary

Subarachnoid CSF

Spinal compartment

Surface coating (“icing”)

Nodular drop metastases

Conceptual dissemination routes; Not an anatomic section

Slide 51 · Two clinical consequences must be kept separate

Clinical consequence

Mechanism

Hydrocephalus

Local blockage of CSF pathways

Leptomeningeal deposits

Tumor-cell spread in subarachnoid CSF

Cauda equina nodules

Distant CSF-associated drop metastases

Hydrocephalus follows obstruction of CSF flow by rapid local growth.

Slide 52 · Medulloblastoma is treatment-sensitive despite high grade

Marked sensitivity to radiation and chemotherapy.

Untreated disease nevertheless has a dismal prognosis.

Therapeutic responsiveness should not be mistaken for indolent behavior.

NATURAL HISTORY

Highly malignant

Poor without treatment

TREATMENT RESPONSE

Radiation-sensitive

Chemotherapy-sensitive

Slide 53 · Prognosis varies markedly by subgroup

WNT-activated cases have favorable overall outcomes.

SHH-activated/TP53-mutant and group 3 tumors are identified as unfavorable groups.

Histologic pattern also contributes to prognostic assessment.

Histologic subtype

Molecular group

Clinical prognosis

Slide 54 · Read the published survival figures as group estimates

Nearly 100% five-year survival for WNT-activated disease.

It contrasts this with approximately 20%–30% survival in more aggressive subtypes.

These are textbook group-level estimates, not an individual prediction or a uniform contemporary cohort.

WNT-ACTIVATED

Nearly 100%

5-year survival reported

AGGRESSIVE SUBTYPES

20%–30%

Range reported

Slide 55 · Treatment intensity is under subtype-based study

Clinical trials explore de-escalation for some more favorable subtypes.

Escalation is explored for biologically adverse disease.

The source describes investigations rather than universal practice recommendations.

Risk stratification

Subtype-adapted trials

Treatment-intensity study

Slide 56 · Targeted therapies remain an investigational direction

Actionable molecular alterations motivate exploration of targeted agents.

The textbook does not specify a universal targeted regimen for medulloblastoma.

“Being explored” must not be presented as established efficacy.

Actionable alteration

Candidate targeted strategy

Clinical investigation

Slide 57 · Application: Rapid cerebellar growth

Illustrative pathology reasoning

Illustrative question: A rapidly enlarging cerebellar tumor impedes CSF passage.

What major complication follows from the mechanical obstruction?

Name the causal chain rather than inferring a molecular subtype.

Teaching answer (speaker discussion)

CSF obstruction → hydrocephalus.

Slide 58 · Application: Pale islands and reticulin

Illustrative pathology reasoning

Illustrative question: A tumor contains pale neuropil-rich islands separated by reticulin-rich cellular zones.

Which histologic pattern is suggested?

Which molecular group is most strongly associated?

Teaching answer (speaker discussion)

Desmoplastic/nodular pattern; Associated with SHH activation.

Slide 59 · Application: Prominent nucleoli and cell wrapping

Illustrative pathology reasoning

Illustrative question: Primitive tumor cells show large irregular vesicular nuclei, nucleoli and cell wrapping.

Which histologic pattern does this constellation support?

Teaching answer (speaker discussion)

Large cell/anaplastic medulloblastoma.

Slide 60 · Application: Interpret a WNT-associated outcome

Illustrative pathology reasoning

Illustrative question: A molecularly confirmed WNT-activated tumor is identified.

What is the direction of the group-level prognostic association?

Why should a precise individual outcome not be promised?

Teaching answer (speaker discussion)

Favorable overall prognosis; Group estimates do not predict a specific patient.

Slide 61 · An integrated pathology interpretation is layered

Establish a cerebellar embryonal tumor with appropriate morphology.

Describe classic, desmoplastic/nodular or large cell/anaplastic features.

Integrate molecular classification and clinical dissemination/risk information.

Anatomic + microscopic pattern

Molecular group

Integrated risk context

Slide 62 · Medulloblastoma: Essential distinctions

Cerebellar malignant embryonal tumor; All are CNS WHO grade 4.

Four molecular groups have different locations and prognoses.

Histologic patterns are recognizable but not sufficient for molecular typing.

The major hazards are CSF obstruction and leptomeningeal dissemination.

Slide 63 · Primary reference

Robbins, Cotran & Kumar. Pathologic Basis of Disease. 11th ed. Chapter 28: The Central Nervous System.

Parent section: Embryonal Tumors; Medulloblastoma, printed p. 1182.

For genuine gross and microscopic examples, see the original textbook Fig. 28.52A–D.