<qa>  What is the safety of combining O6-benzylguanine with temPhase I trial to study the safety of combining O6-benzylguanine with temozolomide in treating children who have recurrent or refractory brain tumors. Drugs used in chemotherapy use different ways to stop tumor cells from dividing so they stop growing or die. O6-benzylguanine may increase the effectiveness of temozolomide by making tumor cells more sensitive to the drug. 