Brain metastasis occurs in 20% to 40% of all cancer patients and impacts quality of life and survival. Stereotactic radiosurgery (SRS), which involves delivering high doses of radiation to the tumour, is the preferred treatment for these patients. It is also associated with a survival advantage compared to irradiation of the whole brain. The main difficulties with SRS are determining whether the tumour is responding early after the treatment and managing radiation‑induced side effects. In previous studies, Dr. Greg Stanisz showed that new medical resonance imaging (MRI) techniques could detect tumour response as early as one week after treatment. For this particular project he will now extend these MRI studies to differentiate between tumour progression and radiation side effects, which can look similar in medical scans but require very different treatments. Patients will be monitored after treatment with a combination of different types of MRI. The main focus of the project is to develop and evaluate new MRI methods to investigate and quantify changes in metabolism and the microenvironment that the tumour undergoes after SRS. These techniques will allow clinicians to identify responding patients within 1 week after treatment and differentiate radiation necrosis from tumour progression. This may help doctors adjust and improve treatment plans. In consequence, treatment for brain metastasis may be faster, more effective and less stressful for patients. This study will also provide researchers with improved tools to evaluate new experimental cancer treatments.