Capecitabine and Radiation Therapy in Treating Young Patients With Newly Diagnosed, Nonmetastatic Brain Stem Glioma or High-Grade Glioma

RATIONALE: Drugs used in chemotherapy, such as capecitabine, work in different ways to stop the growth of tumor cells, either by killing the cells or by stopping them from dividing. Radiation therapy uses high-energy x-rays to kill tumor cells. Capecitabine may make tumor cells more sensitive to radiation therapy. Giving capecitabine together with radiation therapy may kill more tumor cells. PURPOSE: This phase I trial is studying the side effects and best dose of capecitabine when given together with radiation therapy in treating young patients with newly diagnosed, nonmetastatic brain stem glioma or high-grade glioma.