• Histologically confirmed invasive breast cancer

• Metastatic breast cancer

• Oligoprogression defined as one or two distant metastatic lesions, limited to one organ, or the primary tumor or locoregional lymph nodes, increasing ≥20% in size and be larger than 15 mm or if metabolic activity increases (with 20% in SUVmax) on FDG-PET-CT.

• Systemic treatment can be either endocrine, targeted, chemotherapy or immune-checkpoint blockade

• Patients should be on systemic therapy for at least six months. Status should be stable disease or partial or complete response for at least 6 months.

• Oligoprogression has to be detected with radiological imaging comparing the lesion on the same type of imaging modality as has been used at the start of systemic therapy.

• The radiological imaging that shows progression must be performed within 70days prior to LAT.

• Bone metastases are classified as progressive if the lytic component of the lesion increases by ≥20% or the FDG-uptake increases by ≥20% on FDG-PET-CT

• Oligo-progression has to be confirmed with a FDG-PET-CT-scan 5-7 weeks after the initial scan that showed oligoprogression.

• Lesion(s) must be amenable to resection, radiotherapy or radiofrequency ablation with the intent of local obliteration

• Age ≥18

• World Health Organization (WHO) Performance Status 0 or 1

• Signed written informed consent before patient registration according to ICH/GCP, and national/local regulations