الفهرس 
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Abstract
DWI and ADC map provide information on MRI about the cellularity of the tumor and have an important role in the pre-operative differentiation of different tumor types. The present study is conducted on twenty pediatric patients with diagnosed posterior fossa tumor. They were eight males and twelve females. Their age ranged between two years and eighteen years with a mean age of nine years.
Eleven patients were proved to be MB while seven patients were PA.
The aim of this work was to study the role of ADC in differentiation of pediatric intra-axial posterior fossa tumors.
All patients were subjected to full history taking and clinical examination. MRI was performed at 1.5 Tesla MR system using a standard head coil. Imaging included conventional MRI and ADC measurements.
Conventional MRI (T1WI, T2WI, Gd-enhanced) almost gives similar findings in most of posterior fossa tumor pictures. It gives anatomical rather than functional information. It shows precisely the location of the tumor and extension but cannot distinguish between different types of tumors which is based on the tumor cellularity.
By using ADC, we found that the common pediatric brain tumors; PA, MB have specific ADC measurements that can be used to differentiate between them, as they have a strong negative correlation between ADC matrices and the tumor cellularity.
The ADC measurements in cases of MBs ranged from 0.424- 0.890 x10-3 mm2/s, while the ADC measurements in cases of PAs ranged from 1.35 – 2.66 x10-3 mm2/s
Cutoff values of more than 1.3 x10-3 mm2/s that are characteristic of PA and less than 0.9 x10-3 mm2/s of MB seem to provide the diagnosis which may guide further diagnostic studies, treatment planning and outcome.
ADC increases the level of diagnostic confidence in differentiation of pediatric posterior fossa tumors and is able to differentiate the benign PA and the highly malignant MB with CNS dissemination as the later needs totally different treatment strategy. In MB; pre-operatively, it is critical to evaluate the entire neuroaxis to look for disseminated disease. Failure to do so could complicate management decisions and post-operative surveillance imaging of brain and spine and thereby affect the prognosis.
We suggest that ADC is indispensible when evaluating a pediatric patient presenting with a posterior fossa tumor.