Brain metastasis (BrM) develops in approximately 40% of cancer patients. Stereotactic radiosurgery (SRS) is a form of radiotherapy that delivers high-dose per fraction to individual lesions that is commonly used to treat BrM.

Radionecrosis (RN) is an adverse event that occurs in approximately 10 - 25% of patients 6 - 24 months after treatment with SRS. Tumour progression may also occur due to local failure of treatment. Radionecrosis and tumour progression share very similar clinical features including vomiting, nausea, and focal neurologic findings.

Radionecrosis and tumour progression also share overlapping imaging characteristics. Due to their similarities, physicians need to perform a surgical resection to diagnose the complication. By using a hybrid FLT-PET/MRI scan, the investigators propose that this combination scan will provide robust data with which to differentiate between radionecrosis and tumour progression without the need for surgery. The investigators plan to conduct a single center feasibility study to investigate the potential in differentiating between SRS and tumour progression in patients, including those who may have previously undergone SRS for BrM, who are suspected to have either RN or tumour progression using hybrid FLT-PET/MRI imaging.

Inclusion Criteria:

1. 18 years or older
2. Previously treated with SRS for BrM
3. New intracranial lesion with clinical and radiographic findings suspicious for either RN or tumour progression
4. May be planned surgical resection of the lesion in question. The determination that the lesion is appropriate for and may require surgical resection will be made by the multi-disciplinary brain metastasis team. Surgery is preferred but not required. If a patient is planned for surgery, the date does not need to be established and the patient does not need to have consented in order to be eligible for this study, however the imaging procedure will need to occur prior to the date of surgery.
5. A negative serum pregnancy test within the two-week interval immediately prior to PET-MRI imaging for women of child-bearing age
6. Ability to provide written informed concern to participate in the study

Exclusion Criteria:

1. Previous radiotherapy to the intended treatment volume
2. Active malignancy other than sarcoma
3. Inability to remain supine for at least 60 minutes
4. Pregnancy or breast feeding
5. Age <18 years
6. Failure to provide written informed consent
7. Contraindication for MRI as per current institutional guidelines

Patients presenting after being treated with SRS with a new intracranial lesion with clinical and radiographic findings suspicious for either radionecrosis or tumour progression.