1. Tumor Origin

• most arise in the midline cerebellum (inferior vermis)
• obstructive hydrocephalus results from the local extension of the tumor mass from the vermis into the cerebellar hemispheres, 4th ventricle, and brain stem
• tends to be central in children and lateral in adolescence
• predilection for early leptomeningeal spread (especially the lateral form) resulting in a distinct pathologic variant (desmoplastic medulloblastoma or cerebellar sarcoma)

2. Histologic Types

• highly malignant
• tumor is composed of undifferentiated cells thought to arise from primitive neuroepithelial cells in the subependymal zones (external granular layer of the cerebellum)
• considered a primitive neuroectodermal tumor (PNET) because of presence of undifferentiated cells
• average duration of symptoms prior to diagnosis is <2 months due to rapid growth of tumor
• may be associated with abnormalities in chrom. 17

3. Pathology

• may contain focal areas of glial and/or neuronal differentiation
• tumor is vascular and cellular and characterized by deeply-stained nuclei with scant cytoplasm arranged in pseudorosettes

4. Poor Prognosticators

• age < 3 years
• dissemination outside the posterior fossa
• large volume of residual tissue (>25% of original tumor)
• specific deletions on chrom. 17
• survival rates:
  • 50-70% at 5 years
  • 40-50% at 10 years.
  • 60-70% at 5 years. ("good risk" patients)
  • <30% at 5 years. ("poor risk" patients)

1. Primary Tumor

• obstructive hydrocephalus -> increased intracranial pressure (80-90% of patients)
  • infants - increased head circumference, nausea/vomiting, irritability, lethargy
  • children - diplopia, headache, nausea/vomiting, papilledema
• cerebellar signs
  • vermis, lateral, anterior, visual
• others
  • cranial nerve dysfunction, long tract signs

2. Metastases

1. Imaging Studies

2. Pathology

• tumor
• bone marrow - for bony metastases

3. Biologic

• CSF - high polyamine levels, cytology

1. Surgery

• suboccipital craniectomy with debulking by an ultrasound aspirator (vermis), CO2 laser (brain stem)
• mortality - 0%
• morbidity - 10% - infection, neural - ataxia, cranial nerve palsies, etc

2. Craniospinal Irradiation

• 5,000 cGy to the posterior fossa
• 2,500 cGy to the neuraxis (brain, spinal cord)
• standard vs hyperfractionated (bid)
• SE: cognitive disability, growth retardation, endocrine dysfunction
• usually held until child > 3 years of age because of side effects

3. Chemotherapy

• adjuvant - medulloblastomas are highly chemosensitive
• nitrosoureas, platinum cpds, alkylating agents, vincristine
• recommended for high-risk patients only:
  • metastases
  • brain stem involvement
  • partial resection
• may be administered intrathecally