Metastases-directed Radiotherapy in Addition to Standard Systemic Therapy in Patients With Oligometastatic Breast Cancer

Inclusion Criteria: * Metastasized breast cancer - up to 5 clinically manifest (new, progressive, persistent) metastases (a lymph node metastasis and a circumscribed local recurrence are each considered as one metastasis, i.e. also locoregional recurrent breast carcinomas with additional hematogenic metastasis possible) * maximum of 3 cerebral metastases known * indication for palliative drug therapy (endocrine therapy and/or chemotherapy and/or treatment with other substances) given according to guidelines (1st-line or further therapy lines, a special regime is not specified) * Eastern Cooperative Oncology Group (ECOG) Performance Status ≤ 2 * local radiation of all metastases possible * presentation of a written declaration of consent * patient ≥ 18 years Exclusion Criteria: * Previous radiotherapy, if this interferes with treatment within the scope of the study * symptomatic metastases requiring local therapy of all metastases (e.g. pain radiation), a radiation indication (or other local therapy) for individual metastases is not a criterion for exclusion * known central nervous system (CNS) metastasis without extracerebral metastasis (in these cases, immediate local therapy is mandatory) * more than three known CNS metastases (no indication for purely local therapy of only the metastases, primary whole brain radiation is indicated) * multifocal metastasis in one organ with impossibility to comply with the dose constraints for this organ (e.g., no indication for local therapy of only the metastases, primary whole brain radiation is indicated) (e.g. in the liver) * exclusively regional lymph node metastasis without haematogenic metastases (in these cases local therapy is clearly indicated according to guidelines) * relevant comorbidity, if this results in restrictions for further therapy * Incapacity to contract or lack of informed consent * Pregnancy and lactation