Chemotherapy and Radiation Therapy After Surgery in Treating Children With Newly Diagnosed Astrocytoma, Glioblastoma Multiforme, Gliosarcoma, or Diffuse Intrinsic Pontine Glioma

RATIONALE: Drugs used in chemotherapy use different ways to stop tumor cells from dividing so they stop growing or die. Radiation therapy uses high-energy x-rays to damage tumor cells. Giving chemotherapy together with radiation therapy may kill more tumor cells. PURPOSE: This phase II trial is studying how well giving chemotherapy together with radiation therapy after surgery followed by chemotherapy alone works in children with newly diagnosed astrocytoma, glioblastoma multiforme, gliosarcoma, or diffuse intrinsic pontine glioma.