الفهرس 
INTRODUCTION&Aim of the workOnly 14 pages are availabe for public view
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Abstract
Posterior fossa tumors are divided into a matrix of two large categories by location and two large categories by age. The vast I majority of posterior fossa tumors in adults are in the extra-axial location, conversely in childhood; the majority of posterior fossa tumors are in the intra-axial or intraventricular locations. The most common site for posterior fossa extra-axial masses is in the cerebellopontine angle (CPA) cistern. Medulloblastomas have long been recognized as one of the most common tumors in the posterior fossa and account for 4-10% of primary brain tumors, and patients under the age of20account for 15 -20% of CNS tumors, not more than 1 % of adult brain tumors .The cell of origin of medulloblastomas is unknown; the recent theory considers classic medulloblastomas comes from ventricular array and desmoplastic variants from external granular layer. Whereas hemispheric cerebellar involvement is more frequent among adults, vermian involvement is more common among children. Most of posterior fossa tumors present with signs and symptoms of increased intracranial pressure (ICP) due to hydrocephalus, while CPA syndrome is usually due to CPA schwannomas and other tumors arising in this area. The presentation of medulloblastomas in adult is different from that in children owing to the greater incidence of the laterally placed tumors. MRI is the imaging modality of choice in evaluating lesions arising in the posterior fossa. It is the best means of critically localizing lesions and determining the extent of the tumor Considerable contraverse has surrounded the management of