RATIONALE: Radiation therapy uses high-energy x-rays to kill tumor cells. Drugs used in chemotherapy, such as temozolomide and carboplatin, work in different ways to stop the growth of tumor cells, either by killing the cells or by stopping them from dividing. Tamoxifen may make tumor cells more sensitive to radiation therapy and chemotherapy. Giving radiation therapy together with temozolomide, tamoxifen, and carboplatin may kill more tumor cells.