Evaluating AGuIX® Nanoparticles in Combination With Stereotactic Radiation for Brain Metastases

Inclusion Criteria: * Male or female patients aged of at least 18 years on day of signing informed consent. * Histologically-confirmed diagnosis of any histological type of solid tumors, excluding primary central nervous system (CNS) tumors. * Radiological evidence by MRI : At least one and a maximum of 5 brain metastases, and at least one brain lesion with a longest diameter ≥ 2 cm and eligible for FSRT. * Patient without progression on extracranial disease documented by radiological assessment as per RECIST v1.1 within 4 weeks before inclusion. * For patients treated with a systemic anti-cancer therapy: a minimal 2-week washout period is required from the date of last systemic treatment administration to Day 1, except for hormonal agents. * ECOG Performance Status (PS) ≤2. * Absolute neutrophil count (ANC) ≥ 1.0 G/L, Platelets ≥ 75 G/L, Hemoglobin ≥ 8 g/dL, Serum creatinine OR Creatinine clearance according to CKD-EPI ≤ 1.5 x Upper Limit of Normal (ULN) OR ≥ 50 mL/min/1.73m2, ASAT and ALAT ≤ 3 x ULN (or ≤ 5.0 ULN in case of liver metastasis or hepatic infiltration),INR and Activated Partial Thromboplastin Time (aPTT) ≤1.5 x ULN. * Women of child-bearing potential must have a negative serum pregnancy test at screening and must agree to use 2 effective forms of contraception from the time of the negative pregnancy test up to 3 months after the last dose of the study drug. * Fertile men must agree to use contraceptive measures up to 3 months after the last dose of study drug. * Patients who understand, sign, and date the written voluntary informed consent form at the screening visit prior to any protocol-specific procedures. Patient should be able and willing to comply with study visits and procedures as per protocol. * Patients must be covered by a medical insurance. Exclusion Criteria: * Prior local treatment with radiotherapy (whole / partial brain or stereotactic radiosurgery) or surgical resection of brain lesions. * Patient participating to another clinical trial with an investigational agent. * Patients who have not recovered from significant adverse events (i.e. Grade \> 2 AE according to NCI CTCAE v5.0) due to prior treatment with anti-cancer agents with exception of any Grade alopecia or lab values presented in inclusion criteria. * Contra-indication for MRI enhanced with gadolinium (e.g. cardiac pacemaker, implanted defibrillator, certain cardiac valve replacements, certain metal implants). * Patients who are pregnant or breastfeeding.