Patients with oligoprogression of metastatic breast cancer during palliative treatment that is amenable to local therapy will be included. The local ablative therapy (LAT) may consist of stereotactic ablative radiotherapy (SABR), also known as stereotactic body radiation therapy, surgery or radiofrequency ablation (RFA).

Inclusion Criteria:

• 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

Exclusion Criteria:

• Having received more than two lines of systemic therapy for MBC If a treatment regimen has been de-escalated without adding other therapies, this is seen as one line of therapy. For example: Pertuzumab/trastuzumab+docetaxel followed by pertuzumab/trastuzumab will be viewed as one line of systemic therapy.
• Other malignancy except carcinoma in situ and basal-cell and squamous cell carcinoma of the skin, unless the other malignancy was treated ≥5 years ago with curative intent without the use of chemotherapy or radiation therapy
• Current pregnancy or breastfeeding. Women of childbearing potential must use adequate contraceptive protection
• Presence of any medical condition that would place the patient at unusual risk, up to the discretion of the clinician
• Presence of any psychological, familial, sociological, or geographical condition potentially hampering compliance with the study protocol and follow-up schedule

to be determined