Novel Ga68-PSMA PET/CT-tracer to Differentiate Between Radiation Necrosis and Tumor Progression in Brain Metastases

Inclusion Criteria: For all groups: * Written informed consent * Age ≥ 18 years old * WHO PS 0-3 * Measurable lesion ≥10mm according to RANO BM For brain metastases groups: \- newly diagnosed brain metastases from either NSCLC (group 1) or melanoma (group 2) or breast cancer (group 7). For radiation necrosis groups: \- Brain lesion at the location of a formerly brain metastases that has been treated with SRT (\> 9 months ago), with the definite diagnosis of radiation necrosis at the location of formerly SRT-treated brain metastases of NSCLC (group 3), melanoma (group 4) and/ or breast cancer (group 8). For diagnostic dilemma groups: \- Brain lesion at the location of a formerly brain metastases with a diagnostic dilemma of radiation necrosis and recurrent brain metastases of NSCLC (group 5), melanoma (group 6) and/ or breast cancer (group 9). Exclusion Criteria: For all groups: * Known allergy to Ga68-PSMA * Epileptic seizure less than 7 days before Ga68-PSMA PET/CT scan * Life expectancy less than 3 months * Patients with known prostate carcinoma * Pregnancy