Medulloblastoma (PNET arising in posterior fossa tumor)

 

Medulloblastoma (PNET arising in posterior fossa) 

Epidemiology: 

1. (Most common CNS embryonal tumor (> 90% of all cases). 
2. Highly malignant (WHO grade IV). (3) 15–20% of total and ~40% of posterior fossa pediatric brain tumors, rare in adults, most diagnosed by age 5, male female ratio is 3:1.

Presentation/natural history: 

1. Rapid presentation with increased ICP/hydrocephalus, cerebellar signs. 
2. 30% present with CSF mets. 
3. DDX: ependymoma (fourth ventricle), PXA (lateral cerebellar hemisphere), atypical teratoid/ rhabdoid tumor (ATRT) in children < 3 year old.
4. Loss of chromosome 17p in 40–50% of patients. 
5. Expansile, noncalcified mass lesion of the fourth ventricle or cerebellar hemisphere; may have occasional cyst formation; highly cellular and thus shows diffusion restriction on diffusion weighted imaging (DWI).
6. Standard risk: between 3–22 years, no metastases, no gross residual postresection = 5-year survival approaches 100% if ERBB-2 tumor protein negative, 54% if ERBB-2 positive. 
7. High risk: less than 3 years, metastases, residual postresection, 5-year survival ~ 20%.

 Treatment: 

 Surgical excision, adjuvant chemotherapy, craniospinal radiation if age > 3 years.