PET Imaging of Intracranial Cancers With 18F-FSPG

Inclusion Criteria: 1. Written informed consent 2. Participant must be able to complete a PET/CT scan and MRI without the use of sedation 3. Participant has * a primary brain tumor that has been histologically confirmed * OR confirmed or suspected recurrent brain cancer or brain metastasis for which the primary tumor has been histologically confirmed, * OR a brain lesion that is concerning for malignancy for which histopathological confirmation is anticipated following enrollment (e.g., biopsy or surgical resection of the tumor is scheduled) * OR a suspected low-grade brain tumor, where confirmation is based upon a combination of other imaging (e.g. PET/CT, MRI, diagnostic CT) and clinical assessment. * All pathology specimens must be within 1 year of the planned 18F-FSPG PET/CT scan. 4. The time interval between 18F-FSPG PET/CT and other imaging (including other PET/CTs, MRI or diagnostic CT) should be within 4 weeks (exceptions will be allowed for 6 weeks, if there are no other options) 5. Karnofsky Performance Status Scale of 60 or higher OR an ECOG Performance Status of 0-2. 6. No clinically relevant deviations in renal function (Serum Creatinine \> Grade 2 CTCAE v4.0.). Maximal interval between confirmation and injection of 18F-FSPG is one week. 7. No chemotherapy, radiotherapy, or immune/biologic therapy scheduled or performed between other imaging (PET/CTs, MRI, or diagnostic CTs) and18F-FSPG PET/CT. Exclusion Criteria: 1. Patient is scheduled for brain surgery and/or another invasive procedure within the time period of one month prior to 18F-FSPG administration. Minimally invasive needle biopsies are allowed. 2. Known hypersensitivity to the study drug or components of the preparation 3. Patient has completed participation in another clinical study involving administration of an investigational agent in the preceding 4 weeks. However, participation in clinical studies involving other investigational PET or SPECT tracers will not be excluded if in the opinion of the Investigator: * evaluation of 18F-FSPG safety and tolerability will not be confounded by the other investigational PET or SPECT tracer * a minimum of two days (or longer as necessary based on radiological half-life) have elapsed between investigational PET or SPECT tracer administrations to allow acceptable clearance of the tracer * the investigational PET or SPECT tracer administration was well tolerated by the patient. 4. Investigator precludes participation for scientific reasons, for reasons of compliance (e.g., concurrent disease which could compromise the subject's study completion), or for reasons of the patient's safety