The spread of cancer to the brain is referred to as brain metastases. Brain metastases are a common complication of cancer. This study is being done to determine whether the use of a new imaging agent, 18F-fluciclovine, is able to detect which patients are responding to radiation therapy. In addition, this study will look at the changes of the treated brain metastases using this imaging agent over time.

Inclusion Criteria:

1. Male or female, age ≥18 years
2. Performance status, Eastern Cooperative Oncology Group 0-2
3. Confirmed diagnosis of brain metastases with at least one untreated lesion >2 cm in maximum diameter
4. Plan for SSRS per the treating team
5. For women of childbearing potential, a negative serum pregnancy test within 14 days of registration is required
6. For females of reproductive potential: use of highly effective contraception for at least 4 weeks prior to screening and agreement to use such a method during study participation and for an additional 1 week after post-treatment 18F-fluciclovine positron emission tomography/computerized tomography

Exclusion Criteria:

1. Prior anaphylactic reaction to 18F-fluciclovine
2. Evidence of leptomeningeal disease
3. Prior whole-brain radiation therapy
4. Contraindication to MRI (e.g., due to safety reasons, such as presence of a pacemaker)
5. Females pregnant at the expected time of 18F-fluciclovine administration
6. Females who are expecting to be breastfeeding at the time of 18F-fluciclovine and unwilling to stop breast-feeding for 24 hours. Temporary cessation of breastfeeding 24 hours after the time of imaging is allowed
7. Major medical illness or psychiatric/cognitive impairments, which in the investigator's opinion, will prevent completion of protocol therapy and/or preclude informed consent