Stereotactic Brain-directed Radiation With or Without Aguix Gadolinium-Based Nanoparticles in Brain Metastases

This research study is a double-blind, randomized phase II clinical trial of brain-directed stereotactic radiation with or without AGuIX (Activation and Guidance of Irradiation by X-ray) gadolinium-based nanoparticles in the management of brain metastases at higher-risk of local recurrence with radiation alone. The study agent (AGuIX gadolinium-based nanoparticles) has two main parts. The first is gadolinium, also known as "contrast," which is typically injected into a vein during a MRI scan. The second part is a nanoparticle (linked to the gadolinium) that may make focused radiation work more effectively. The AGuIX gadolinium-based nanoparticles are deposited within the brain metastases via the bloodstream and then the brain metastases are radiated. The U.S. Food and Drug Administration (FDA) has not approved AGuIX gadolinium-based nanoparticles as a treatment for any disease. AGuIX gadolinium-based nanoparticles have been tested in other studies for safety and efficacy in patients with brain metastases who are also receiving radiation. This study seeks to determine whether AGuIX gadolinium-based nanoparticles improve outcomes relative to placebo.Participants will be "randomized" into one of the study groups: * Group A: Radiation plus AGuIX gadolinium-based nanoparticles * Group B: Radiation plus placebo The research study procedures include screening for eligibility and study treatment including evaluations and follow up visits. TherAguix, the manufacturer of the AGuIX gadolinium-based nanoparticles, is supporting this research study by providing the AGuIX gadolinium-based nanoparticles that will be evaluated in this study. TherAguix is also covering the cost of the study. It is expected that about 134 people will take part in this research study.