Combination Chemotherapy Followed by Radiation Therapy in Treating Patients With Medulloblastoma, Supratentorial Primitive Neuroectodermal Tumor, or Ependymoma

brain and central nervous system tumors, childhood infratentorial ependymoma, childhood supratentorial ependymoma, adult medulloblastoma, adult myxopapillary ependymoma, adult anaplastic ependymoma, adult ependymoblastoma, untreated childhood supratentorial primitive neuroectodermal tumor, untreated childhood medulloblastoma, newly diagnosed childhood ependymoma, adult supratentorial primitive neuroectodermal tumor (pnet), interventional