FDG-PET/CT vs. CT for Monitoring Metastatic Breast Cancer

Inclusion Criteria: 1. Women and men aged ≥18 years 2. Diagnosis of distant relapsed MBC (biopsy-verified) or de novo breast cancer. In patients with distant relapsed MBC, biopsy verification from a distant metastasis is required. In patients with de novo MBC, biopsy verification of primary tumor and diagnostic imaging with distant metastasis with a typical pattern of MBC is required. 3. Considered eligible for first-line systemic treatment 4. Considered eligible for continuous treatment monitoring by scans. 5. Signed informed consent 6. Participants must have the ability to read and understand the following languages based on their country of participation: in Denmark, patients must be able to read and understand Danish; in Italy, they must be able to read and understand Italian or English; and in Germany, they must be able to read and understand German or English. In case of patients for whom it is necessary to start first-line systemic treatment while still waiting for the evaluation of the biopsy, it is allowed to include the patients, as long as the other criteria are fulfilled and the biopsy is made or planned. In case verification by biopsy fails, the patients will leave the trial (cf. 4c). We expect that up to 3% of the patients included will start first-line systemic treatment prior to evaluation of the biopsy Exclusion Criteria: 1. Pregnant or lactating women 2. Ongoing oncological treatment for another cancer 3. Exclusively brain metastasis 4. Allergy to FDG