RAPid SimPLE Targeted Radiation Treatment for Brain Metastases

Inclusion Criteria: Subjects must meet all of the following criteria to be eligible for participation in this study: * Age ≥ 18 * Pathological diagnosis of a non-hematopoietic malignancy * Brain metastases of any size * Any number of untreated or progressing, previously treated brain metastases that can all be contoured and targeted * Presence of extracranial disease * Diagnosis-Specific Graded Prognostic Assessment ≤ 2.0. (https://brainmetgpa.com/) or Graded Prognostic Assessment ≤ 2.0 (Appendix I) * Able to complete the EuroQOL (EQ-5D-5L) questionnaire * Willing and able to have regular imaging follow up * Feasible to start protocol treatment within 14 days of participant enrolment * Karnofsky Performance Score (KPS) ≥ 50 Exclusion Criteria: Subjects are excluded from the study if any of the following criteria apply: * Inability to have a brain MRI * WBRT less than 3 months prior to randomization * Disseminated leptomeningeal carcinomatosis (limited pachymeningeal disease is permitted) * Multiple sclerosis * Neurologically declining despite corticosteroids * Appropriate for surgery or stereotactic radiosurgery * Germ cell and primary brain tumours * Systemic lupus erythematosis, scleroderma, or other connective tissue disorders not in remission * Any other serious intercurrent illness or medical condition judged by the local investigator to compromise the participant's safety, preclude safe administration of the planned protocol treatment, or prevent the participant from being managed according to the protocol guidelines * Pregnancy * Potentially fertile men or women of childbearing potential who are unwilling to employ highly effective contraception