COntinue the SaMe Systemic Therapy After Local Ablative Therapy for Oligo Progression in Metastatic Breast Cancer - the COSMO Study

Inclusion Criteria:

* Histologically confirmed invasive breast cancer
* Metastatic breast cancer
* o Oligoprogression is defined as an increase of ≥20% in size or, in case of FDG-PET-CT, metabolic activity (≥20% in SUVmax) involving a maximum of two lesions. These lesions may consist of: 1 or 2 distant metastatic lesions limited to 1 organ, the primary tumor or locoregional lymph nodes.
* 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.
* Lesion(s) must be amenable to resection, radiotherapy or percutaneous ablation with the intent of local obliteration.
* The radiological imaging that shows progression must be performed within 70 days 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
* Lymph nodes should be larger than 15 mm upon first detection of oligoprogression to be classified as progressive.
* Oligo-progression has to be confirmed with a FDG-PET-CT-scan 5-7 weeks after the initial scan that showed oligoprogression.
* 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