 - Pre-treatment laboratory tests for patients receiving 18FFLT must be performed within 21 days prior to study entry. These must be less than 2.5 times below or above the upper or lower limit range for the respective laboratory test for entry into the study. In those instances where a baseline laboratory value is outside of this range, then such a patient will be ineligible for enrollment. For the followup scanning sessions after therapy has been instituted, laboratory testing will also be required due to the use of FLT. The patients have brain tumors and will receive various forms of therapy; therefore many routine laboratory tests may not be within the typical normal range. As such, a factor of 4.0 times above or below the upper or lower value for the normal range for any laboratory test will be used to determine the acceptable range for the 2nd and possible 3rd imaging timepoints. The baseline laboratory testing will include liver enzymes ALT, AST, ALK, LDH, bilirubin total, serum electrolytes, CBC with platelets and absolute neutrophil counts, prothrombin time, partial thromboplastin time, BUN, creatinine. Previous urinalysis abnormalities will not preclude the patient from being studied. For those patients receiving coumadin or another anticoagulant the upper limit for prothrombin time or partial thromboplastin time must not exceed 6 times the upper limit of the normal range. 