RecruitingPhase 2Radiation therapyStereotactic Brain-directed Radiation With or Without Aguix Gadolinium-Based Nanoparticles in Brain MetastasesThe purpose of this study is to determine whether AGuIX (Activation and Guidance of Irradiation by X-ray) gadolinium-based nanoparticles make radiation work more effectively in the treatment of patients with brain metastases that are more difficult to control with stereotactic radiation alone.