• The T1-weighted axial MRI (Fig. 21.1A) shows a lesion involving the midline posterior fossa and compression of the 4th ventricle (vermis lesion).
  
• The T2-weighted sagittal MRI (Fig. 21.1B) demonstrates the same lesion in the vermis and hydrocephalus.

• Differential diagnosis for a single posterior fossa lesion in a young adult includes^1,2
  
  
  
  • Metastasis (most common brain lesion in an adult)
    
  • Hemangioblastoma (accounts for 7–12% of posterior fossa tumors)
    
  • Pilocytic astrocytoma (could be solid or cystic form)
    
  • Medulloblastoma (more common in childhood. There is a second peak between 20–40 years. It accounts for 5% of all adult posterior fossa tumors and 1% of all adult brain tumors. Medulloblastoma in adulthood is more likely laterally located; in childhood, it tends to be in the midline.)
    
  • Other less likely lesions include tuberculoma, cavernous hemangioma, brain abscess, high-grade glioma, lymphoma, dermoid, infarction, hemorrhage, etc.

• The tumor is located in the vermis, just posterior to the 4 th ventricle with signs of hydrocephalus (both radiographically and clinically).
  
• Initial management³
  
  
  
  • The patient should be admitted to the intensive care unit.
    
  • Place the patient on steroids (dexamethasone 10 mg intravenous (i.v.) once, the 6 mg i.v. every 6 hours) with histamine blockers for gastric ulcer prevention.
    
  • Obtain a basic preoperative laboratory panel: complete blood count, electrolytes, prothrombin time/partial thromboplastin time, type and crossmatch 2 units of packed red blood cells.
    
  • Due to the presence of hydrocephalus, surgical intervention should be performed urgently (within a few hours).
    
  • If possible, also obtain preoperative brain and spine MRIs with contrast to better delineate the tumor and its vasculature and to rule out drop metastases
  
  
• Definitive treatment should include surgical removal of the lesion with likely temporary external ventricular drain to control the intracranial pressure and anticipate postoperative cerebellum swelling.³
  
  
  
  • Posterior fossa craniotomy with prone positioning (Concorde) and midline incision
    
  • May place Frazier burr hole for ventricular catheter (3 to 4 cm from the midline and 6 cm above the inion)
    
  • Adjuncts to surgery should include
    
    
    
    • Microscope
      
    • Intraoperative ultrasound
      
    • If available – frameless navigation
  
  
• The histopathology of the tumor will guide further treatment plan.

• Metastatic disease: whole brain radiation 45–50 Gy plus a boost to the tumor bed to bring the total treat ment up to 55 Gy, all with low fractions of 1.80–2.0 Gy. Possible combination with chemotherapy for certain entities such as small cell lung carcinoma, adenocarcinoma of colorectal, etc. A workup for the primary tumor is also recommended (such as CT thorax, abdomen and pelvis, mammogram, and serum tumor markers).³
  
• Medulloblastoma: often spreads through the subarachnoid space and the tumors are radiosensitive. Radiotherapy of the entire neuraxis is considered standard, even if no obvious lesions are present on postoperative imaging. Best survival rates are obtained with 3600 to 4000 cGy to the whole craniospinal axis, supplemented to 5400 – 5600 cGy at the primary site.³
  
• Hemangioblastoma, cavernoma, and pilocytic astrocytoma should be completely removed as a definitive treatment. Adjuvant therapy remains controversial with this diagnosis.³
  
• Tuberculoma: It is not necessary to remove completely. It requires postoperative antituberculosis treatment: rifampicin 450–600 mg/day (10 mg/kg), isoniazid 300–450 mg/day (3–10 mg/kg), ethambutol 15 mg/kg/day, pyrazinamide 20–30 mg/kg/day. All these medications are given orally every day for the initial 3–4 months. Then the first two medications are continued (rifampicin and isoniazid) for 16–18 months. Pyridoxine (10 mg/day) is invariably added to prevent peripheral neuropathy due to isoniazid. Streptomycin could be added for the initial treatment, given intramuscularly 1 g/day (20–25 mg/kg).

• Table 21.1 summarizes the differences between medulloblastoma in childhood and adulthood.^4,5

Syndrome associated with medulloblastoma^4,5

• Gorlin syndrome (multiple nevoid basal cell carcinomas)
  
• Rubinstein–Taybi syndrome
  
• Ataxia-telangiectasia
  
• Turcot’s syndrome (polyposis-glioma)
  
• Li–Fraumeni syndrome (germ line TP53)
  
• Neurofibromatosis
  
• Tuberous sclerosis

• The prognosis for patients with medulloblastoma is related to⁵
  
  
  
  • Tumor size: Large tumors have been shown to have a lower 5-year disease-free survival rate.
    
  • Invasiveness of the tumor to surrounding structure such as brainstem could prevent complete removal of the tumor.
    
  • Dissemination of the tumor through the neuraxis correlates with poor outcome.
    
  • Age of patients: younger age is much more likely to have dissemination.
    
  • Postoperative residual tumor correlates with poor outcome.
  
  
• Overall, with surgery and radiation, the 5-year disease-free survival rate could approach 80%.⁵

• Current staging for medulloblastoma divides the risk into standard and poor risk based on several factors, such as extent of tumor, resection, age, and histology.^5,6
  
• Table 21.2 summarizes the factors affecting survival in medulloblastoma.⁵