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

Inclusion Criteria: * Participants must have a biopsy proven solid malignancy and at least one intracranial measurable lesion spanning ≥5mm in maximal unidimensional size and radiographically consistent with or pathologically proven to be a brain metastasis AND meet one of the following additional criteria regarding the primary site or nature of the intracranial disease: * Melanoma with intracranial growth consistent with tumor progression despite immunotherapy * Gastrointestinal primary * HER2 positive breast cancer (subtype assessed using most representative tissue available in opinion of enrolling clinician and/or study PI) * Cystic metastases * Metastases ≥2cm in maximal unidimensional size * Locally recurrent metastases after prior stereotactic radiation * Locally recurrent metastases after prior whole brain radiation \*Patients with metastases from melanoma, GI primaries, or HER2+ breast cancer, as well as those with cystic metastases or metastases ≥2cm in maximal unidimensional size, who have local recurrences after prior brain-directed radiation can only be treated in the strata permitting prior radiation (last two strata above) * Age ≥18 years at diagnosis of brain metastases * Estimated glomerular filtration rate of ≥ 60 mL/min/1.73m2 * Karnofsky performance status of at least 70 (i.e. at minimum, "cares for self" but "unable to carry on normal activity or do active work") * Estimated survival based on extracranial disease of at least 3 months in the…