Observation or Radiation Therapy With or Without Combination Chemotherapy in Treating Patients With Low-Grade Glioma

RATIONALE: Radiation therapy uses high-energy x-rays to damage tumor cells. Drugs used in chemotherapy use different ways to stop tumor cells from dividing so they stop growing or die. It is not yet known whether radiation therapy combined with chemotherapy is more effective than radiation therapy alone in treating patients with low-grade glioma. PURPOSE: Phase II/III trial to evaluate observation and to compare the effectiveness of radiation therapy with or without combination chemotherapy in treating patients with low-grade glioma.