RATIONALE: Drugs used in chemotherapy, such as carboplatin, vincristine, etoposide, and cytarabine, 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. Giving chemotherapy with or without radiation therapy after surgery may kill any tumor cells that remain after surgery. Sometimes, after surgery, the tumor does not need more treatment until it progresses. In this case, observation may be sufficient.