2. Drugs that reduce dasatinib exposure such as H2 blockers or proton-pump inhibitors (eg famotidine and omeprazole), which can cause long-term suppression of gastric acid secretion. The concomitant use of H2 blockers or proton pump inhibitors with dasatinib is in general not recommended and antacids should be considered in place of H2 blockers or proton pump inhibitors in patients receiving dasatinib therapy. However, given that nearly all recurrent malignant brain tumor patients are on dexamethasone for increased intracranial pressure, such patients must also receive effective medical therapy to prevent complications related to increased gastric acid secretion due to chronic dexamethasone therapy. Therefore all patients enrolled on the current protocol will receive standard H2 blocker (preferred) or proton pump inhibitor (PPI) therapy to be administered on a daily basis each evening. Dasatinib will be administered each morning in order to maximize the time interval from administered H2 blocker (preferred) or proton pump inhibitor (PPI).