• 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 opinion of the enrolling clinician and/or study PI

• Ability to understand and the willingness to sign a written informed consent document

• The effects of AGuIX on the developing human fetus are unknown. For this reason, women of child-bearing potential must agree to use adequate contraception prior to study entry and for the duration of the therapeutic component of study participation