Medulloblastoma (Posterior Fossa Midline Mass & Molecular Staging)

• Seen in children (peak age 3–8 years, male:female 1.5:1, the most common malignant pediatric brain tumor, accounting for 20% of all childhood CNS neoplasms; second smaller adult peak at 20–30 years). Presents with progressive morning headaches, recurrent non-bilious projectile vomiting, truncal ataxia, broad-based unsteady gait, and sixth nerve (abducens) palsy due to elevated intracranial pressure and fourth ventricular obstruction.
• Caused by highly aggressive WHO Grade 4 embryonal neuroepithelial tumor arising within the posterior fossa (vermis/roof of fourth ventricle in classic childhood subtypes; cerebellar hemispheres in older children/adults):
• (1)
  Histopathology: Densely packed sheets of small, round, hyperchromatic blue cells with high nuclear-to-cytoplasmic ratio, brisk mitoses, apoptosis, and neuroblastic Homer Wright rosettes.
• (2)
  WHO Molecular Consensus Subgroups:
• 1.
  WNT-Activated (10%): Best prognosis (long-term survival >95%); classic histology; typically located in CP angle / cerebellar peduncle; CTNNB1 mutations and Monosomy 6.
• 2.
  SHH-Activated (30%): Intermediate prognosis; bimodal age (infants and adults); desmoplastic/nodular histology; laterally located in cerebellar hemispheres; TP53 status dictates outcome.
• 3.
  Group 3 (MYC-Amplified, 25%): Worst prognosis; infants/children; classic or large cell/anaplastic histology; high rate of leptomeningeal drop metastases at presentation (>40–50%).
• 4.
  Group 4 (Non-WNT/Non-SHH, 35%): Intermediate prognosis; older children (males > females); midline fourth ventricle; frequent isochromosome 17q.

Imaging Findings

• Magnetic Resonance Imaging (Brain MRI - Diagnostic Gold Standard Framework):
• –
  Hallmark Midline Posterior Fossa Mass: Solid, well-demarcated mass centered directly in the cerebellar vermis, projecting into and expanding the fourth ventricle from its roof (superior medullary velum).
• –
  Restricted Diffusion (High Cellularity Signature):
• 1.
  Profound, homogeneous restricted diffusion (intensely bright on DWI b=1000, profoundly dark on ADC maps), directly reflecting dense, packed sheets of small round blue cells with minimal extracellular water.
• –
  T1WI & T2WI Characteristics: Hypointense to isointense to gray matter on T1WI; intermediate-to-low signal on T2WI (due to high nuclear density), surrounded by variable vasogenic edema.
• –
  Post-Contrast Enhancement: Heterogeneous, moderate-to-intense enhancement post-gadolinium in >90% of cases.
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  Whole-Spine MRI Screening (The 'Drop Metastasis' Protocol):
• 1.
  Mandatory, urgent contrast-enhanced whole-spine MRI prior to surgery to screen for leptomeningeal drop metastases (nodular 'sugar-coating' or discrete intradural extramedullary enhancing deposits along the cauda equina and cord surface).
• –
  DWI / ADC (Pathognomonic Discriminator): Demonstrates avid, homogeneous diffusion restriction (bright on DWI, very dark on ADC <600–700 × 10⁻⁶ mm²/s) due to densely packed small round blue cells—immediately distinguishing medulloblastoma from juvenile pilocytic astrocytoma.
• –
  **SWI / T2* GRE: Punctate blooming artifact** from small intratumoral microhemorrhages or fine calcifications (present in ~20%).
• Computed Tomography (CT Head - Unenhanced Protocol):
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  Characteristically hyper-attenuating / hyperdense (45–65 HU) relative to normal cerebellar cortex on unenhanced CT, with internal microcalcifications in up to 20%.
• Plain Radiograph (X-Ray) & Bone Findings:
• –
  Pediatric Skull X-Ray: Signs of obstructive hydrocephalus and chronic raised intracranial pressure, including widening (diastasis) of the cranial sutures, 'copper-beaten' (beaten brass) skull markings, and erosion of the dorsum sellae.

Differential Diagnosis

Important Points to Remember

• Diagnostic Pitfall: Operating on or discharging a suspected Medulloblastoma patient without obtaining a complete pre-operative contrast-enhanced whole-spine MRI! Medulloblastoma has an alarming propensity for CSF drop dissemination (Chang Staging: M0 to M3). Once surgical craniectomy is performed, post-operative blood products, surgical debris, and reactive leptomeningeal inflammation mimic drop metastases on spinal MRI for weeks. Pre-operative contrast-enhanced whole-spine MRI is mandatory and changes management from localized therapy to craniospinal axis radiation.
• Crucial Review Areas:
• 1.
  Fourth ventricular roof (superior medullary velum) versus floor to differentiate medulloblastoma from ependymoma.
• 2.
  ADC map: profoundly low ADC values (<700) confirming hypercellular small round blue cell tumor.
• 3.
  Entire spinal cord and cauda equina on post-contrast sagittal/axial T1WI for nodular 'sugar-coating' drop metastases.
• 4.
  Lateral and third ventricles for acute obstructive hydrocephalus and transependymal interstitial edema.
• Radiologist Safe Practice & Management:
• 1.
  Immediate Neurosurgical Alert: Direct telephone notification to the Pediatric Neurosurgical Consultant; emergency external ventricular drain (EVD) or endoscopic third ventriculostomy (ETV) if acute obstructive hydrocephalus is present.
• 2.
  Complete Neuraxis Staging: High-resolution post-contrast whole-spine MRI (prior to surgery) and diagnostic CSF cytology (collected 14 days post-op via lumbar puncture).
• 3.
  Maximal Safe Surgical Resection: Gross total resection (GTR) or near-total resection (<1.5 cm2 residual tumor) through a suboccipital craniectomy.
• 4.
  Comprehensive Molecular Classification: Mandatory immunohistochemistry and molecular profiling (WNT, SHH, Group 3, Group 4, and TP53 status) to risk-stratify adjuvant therapy.
• 5.
  Risk-Adapted Adjuvant Therapy: Craniospinal irradiation (CSI) followed by multi-agent maintenance chemotherapy (Cisplatin, Lomustine, Vincristine, Cyclophosphamide) in children >3 years (sparing radiation in children <3 years to prevent devastating neurocognitive impairment).