PRIOR CONCURRENT THERAPY: Biologic therapy: No biologic therapy for at least 4 weeks after study completion No immunotherapy for at least 4 weeks after study completion Chemotherapy: No chemotherapy for at least 4 weeks after study completion Endocrine therapy: Concurrent steroids allowed Radiotherapy: See Disease Characteristics No prior radiotherapy for this disease or other brain tumor No prior radiotherapy to face, head, or neck Surgery: See Disease Characteristics Recovered from prior surgery or postoperative complication Other: At least 48 hours since prior MRI scan with contrast No concurrent MRI scans with contrast No other concurrent experimental drug