The purpose of this study is to evaluate the clinical usefulness of dynamic [18F]FDG PET imaging in assessing brain metastasis post stereotactic radiotherapy to separate true progression from the treatment-related changes.

After being informed about the study and potential risk, all participants giving written informed consent will be screened to determine if participant being referred fits eligibility criteria for the study, under of the supervision of the radiation oncologist. If a participant agrees to participate in this study, they will receive three additional imaging sessions (the dynamic PET scan). The first session will occur prior to starting treatment; the second session will occur at about 10 weeks post treatment; the third session will occur at first concern of progression at treated lesions or 12 months post-treatment, whichever comes earlier. These scans will be scheduled in addition to the participant's regular follow-up scans and appointments.

Inclusion Criteria:

• Age 18 or older.
• Willing and able to provide consent.
• ECOG 0-2.
• Life expectancy ≥12 months.
• Creatinine clearance ≥30 ml/min within 28 days prior to registration.
• At the discretion of the treating oncologist, it is believed that at least 1 lesion (or resection cavity) is best managed with SRS/SFR

Exclusion Criteria:

• Pregnant or nursing women.
• Men or women of childbearing potential who are unwilling to employ adequate contraception.
• Inability to complete a brain MRI.
• Known allergy to gadolinium.
• Inability to complete a PET scan.
• Uncontrolled diabetes mellitus.
• Primary germ cell tumour, primary CNS tumour, or lymphoma
• SRS/FSRT is no longer indicated for any BMs
• Must be able to read and write English