Medulloblastoma is a highly malignant primary brain tumor that originates in the cerebellum or posterior fossa. Tumors that originate in the cerebellum are referred to as infratentorial because they occur below the tentorium, a thick membrane that separates the cerebral hemispheres of the brain from the cerebellum. Another term for medulloblastoma is infratentorial primitive neuroectodermal tumor (PNET). Medulloblastoma is the most common PNET originating in the brain. The majority of this tumors are invasive and rapidly growing tumors that, unlike most brain tumors, spread through the cerebrospinal fluid (CSF) and frequently metastasize to different locations in the brain and spine.