This is a case of a common pediatric tumor being found in an uncommon location.  In pediatric populations, ependymomas are usually found in the posterior fossa.  And, they are not usually amongst the differential of a pineal region mass. 

However, the mass does abut the posterior portion of the third ventricle and it is its location within the 3rd ventricle and its subsequent expansion of the ventricle and obstruction of the aqueduct that should aid diagnosis.