Study patients: Adult patients (n = 20) with compelling MRI evidence of primary brain tumor that have not yet undergone surgery or any tumor-directed therapy. Four to eight patients per year will be enrolled during the first 3 years, for a total of 20 patients. Enrollment will be weighted toward lower-grade tumors during year 1 in order to allow for longer followup times during the life of the project. Dr. Glantz (Huntsman Cancer Institute Neuro-oncologist) will identify and recruit patients for participation, and obtain informed consent.

Inclusion Criteria:

• Adult patients with compelling evidence of primary brain tumor based on clinical and MRI imaging characteristics that have not yet received surgery, histological diagnosis, or any tumor-directed therapy.

Such evidence will include: MRI or CT scan-documented mass lesion within the brain, accompanied by anatomically appropriate neurological signs and symptoms, in the absence of a probable competing diagnosis such as brain abscess or primary intracranial hematoma.

• Patients must be 18 years or older for inclusion in this study. There is little experience with the safety of [18F]FLT in children, and the risks associated with radiation exposure may be increased for children under 18 years old as well.
• Karnofsky performance status > 60%.
• Patients must document their willingness to be followed for at least 24 months after recruitment by signing informed consent documenting their agreement to have clinical endpoints and the results of histopathologic tissue analysis (when tissue becomes available from routine care) entered into a research database.
• All patients, or their legal guardians, must sign a written informed consent and HIPAA authorization in accordance with institutional guidelines.
• Determination of pregnancy status: Female patients that are not postmenopausal or surgically sterile will undergo a serum pregnancy test prior to each set of multi-tracer PET scans. A negative test will be necessary for such patients to undergo research PET imaging.
• Pre-treatment laboratory tests for patients receiving [18F]FLT 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 4.0x 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 3rd imaging timepoints. The 4.0x value will be critical for liver function test results which can be extremely variable, and a 4.0x above the upper normal range is still acceptable for continued inclusion in this study. The baseline laboratory testing will include liver enzymes (SGOT, SGPT, ALK Phos, GGT, LDH), bilirubin (direct and total), amylase, serum electrolytes, CBC with platelets and absolute neutrophil counts, prothrombin time, partial thromboplastin time, BUN, creatinine, and urinalysis. For the assessments at subsequent imaging time points the urinalysis will not be required however the remainder of the laboratory tests as described for baseline assessment will again be obtained. A negative HIV test will required at baseline. This is due to the previous toxicity of FLT noted in this patient group.

Exclusion Criteria:

• Patients with clinically significant signs of uncal herniation, such as acute pupillary enlargement, rapidly developing motor changes (over hours), or rapidly decreasing level of consciousness, are not eligible.
• Patients with known allergic or hypersensitivity reactions to previously administered radiopharmaceuticals.

Patients with significant drug or other allergies or autoimmune diseases may be enrolled at the Investigator's discretion.

• Patients who are pregnant or lactating or who suspect they might be pregnant. Serum pregnancy tests will be obtained prior to each set of multi-tracer PET scans in female patients that are not postmenopausal or surgically sterile.
• Adult patients who require monitored anesthesia for PET scanning.
• HIV positive patients due to the previous toxicity noted with FLT in this patient group.
• Patients who have undergone surgery or receive any previous tumor-directed therapy for their brain tumor.