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Medulloblastoma

Summary

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  • Most common malignant brain tumour in children
  • Arises from primitive neuroectodermal cells in the cerebellum
  • Typically presents as a midline posterior fossa mass with characteristic imaging features

Pathophysiology

  • Originates from granule neuron precursor cells in the external granular layer of the cerebellum
  • Four molecular subgroups: WNT, SHH, Group 3, and Group 4
  • Genetic alterations include:
    • PTCH1 and SUFU mutations in SHH subgroup
    • CTNNB1 mutations in WNT subgroup
    • MYC amplification in Group 3
    • SNCAIP duplication in Group 4

Demographics

  • Peak incidence: 3-7 years of age
  • Male to female ratio: 1.5:1
  • Accounts for 20% of all paediatric brain tumours
  • Less common in adults, representing <1% of adult brain tumours

Diagnosis

  • Clinical presentation:
    • Increased intracranial pressure: headache, vomiting, lethargy
    • Cerebellar dysfunction: ataxia, dysmetria, nystagmus
    • Cranial nerve palsies (VI, VII)
  • Laboratory findings:
    • CSF cytology may show tumour cells
    • Elevated CSF protein levels
  • Histopathology:
    • Small, round, blue cells with high nuclear-to-cytoplasmic ratio
    • Homer Wright rosettes may be present

Imaging

  • CT:
    • Hyperdense, well-circumscribed posterior fossa mass
    • Often associated with hydrocephalus
    • Calcifications in 10-20% of cases
  • MRI:
    • T1: hypointense to isointense
    • T2: heterogeneous, often hyperintense
    • FLAIR: heterogeneous signal
    • DWI: typically shows restricted diffusion
    • Contrast-enhanced T1: heterogeneous enhancement
    • "Drooping cerebellar tonsils" sign may be present
  • Spine imaging:
    • Essential to evaluate for leptomeningeal spread

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  • A 20-year-old patient presented with headache.
  • A well-circumstribed non-enhancing lesion arising from the roof of the fourth ventricle.
  • The hyperdensity on CT and low values on ADC indicated hypercellularity.

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  • A hyperdense lesion filling the fourth ventricle with diffusion restriction but no enhancement.
  • Immunohistochemistry revealed a non-WNT and non-SHH medulloblastoma.

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  • A 25-year-old patient presented with headache.
  • MRI showed a well demarcated non-enhancing lesion in the fourth ventricle causing hydrocephalus.
  • Diffusion restriction and hyperdensity on CT indicated hypercellularity.
  • 6 weeks after proton beam therapy, the patient developed upper and low limb weakness and sensory disturbance.
  • MRI showed a progressively enlarging enhancing lesion at C1 and C2 and extensive oedema throughout the cord.
  • DCE perfusion revealed an elevated extra-cellular volume and a Type 1 curve and ADC values were not low, which was most consistent with treatment-related changes (rather than progressive disease).
  • On imaging 2 months later after steroid therapy, the enhancing lesion regressed (not shown).

Treatment

  • Maximal safe surgical resection
  • Risk stratification based on age, extent of resection, and metastatic status
  • Adjuvant therapy:
    • Standard-risk patients: craniospinal radiation (23.4 Gy) with posterior fossa boost (55.8 Gy) and chemotherapy
    • High-risk patients: higher dose craniospinal radiation (36-39 Gy) with posterior fossa boost and intensified chemotherapy
  • Molecular subgroup-specific targeted therapies under investigation:
    • SMO inhibitors for SHH subgroup
    • WNT pathway inhibitors for WNT subgroup
  • Long-term follow-up for neurocognitive and endocrine sequelae

Differential diagnosis

Differential Diagnosis Differentiating Feature
Ependymoma More likely to occur in the fourth ventricle; may have calcifications
Pilocytic Astrocytoma Typically cystic with enhancing mural nodule; less likely to have drop metastases
Atypical Teratoid/Rhabdoid Tumour More aggressive clinical course; often in children <3 years old
Choroid Plexus Papilloma Typically arises from choroid plexus; enhances more homogeneously
Brainstem Glioma Primarily involves the brainstem; less likely to have cerebellar involvement
Cerebellar Hemangioblastoma Usually has associated cyst; intense nodular enhancement
Metastasis Multiple lesions; ring or nodular enhancement; grey-white junction predilection; no intrinsic fourth ventricular origin
Cerebellar Abscess Smooth thin ring enhancement; restricted central DWI; surrounding oedema; no fourth ventricular involvement
Diffuse Midline Glioma Typically involves the brainstem; less likely to have cerebellar involvement
Cerebellar Infarction Follows vascular territory; diffusion restriction on MRI