Imaging Features for the Risks for Recurrence After Stereotactic Radiosurgery in Brain Metastasis

Inclusion Criteria: 1. Patients who underwent stereotactic radiosurgery (SRS, gamma-knife radiosurgery or cyberknife radiosurgery) for brain metastases 2. Patients with lesions eligible for SRS : * One to ten newly diagnosed brain metastases * Patients without acute neurological symptom 3. Patients with a Karnofsky performance status score of 70 or higher 4. Patients who underwent brain MRI within 1 month of enrollment 5. Patients with measurable enhancing lesions on MRI. 6. Patients who have available reference standard (second-look surgery for recurrence) or available follow up imaging for clinic-radiologic reference standard. 7. A longest diameter \> 1.5 cm for tumor habitat analysis. Exclusion criteria: 1. Patients who have undergone prior brain surgery, SRS, or whole-brain radiation therapy. 2. Patients who are diagnosed with leukemia, lymphoma, germ-cell tumor, small-cell lung cancer, leptomeningeal disease, or unknown primary tumor. 3. Patients with age \< 18 years. 4. Patients without baseline MRI. 5. Patients with nonmeasurable enhancing lesions on MRI : all other lesions, including lesions with longest dimension \< 10 mm, lesions with borders that cannot be reproducibly measured, dural metastases, bony skull metastases, and leptomeningeal disease.