Tipifarnib may stop the growth of tumor cells by blocking the enzymes necessary for their growth. Radiation therapy uses high-energy x-rays to damage tumor cells. Tipifarnib may make tumor cells more sensitive to radiation therapy. Combining tipifarnib with radiation therapy may kill more tumor cells. This phase I/II trial is studying the side effects and best dose of tipifarnib to see how well it works when given together with radiation therapy in treating young patients with newly diagnosed brain stem glioma. (Phase I closed to accrual as of 1/19/06)