Randomized phase 2 trial to evaluate the efficacy of local therapy for oligometastasis from ER/PR-positive breast cancer. The study hypothesis is that local therapy in addition to systemic therapy improves progression-free survival in comparison with systemic therapy alone.

Patients with estrogen receptor/ progesterone receptor-positive oligometastatic breast cancer with disease controlled after at least six months of systemic therapy will be enrolled in the study. Patients will be randomized to receive local therapy for oligometastatic sites in addition to systemic therapy or systemic therapy alone. Local therapy strategies will include surgery, radiotherapy, and radiofrequency ablation.

Inclusion Criteria:

• Female sex
• ≥ 18 years of age
• Histologically confirmed invasive breast cancer, with oligometastatic disease defined as one of the following criteria: 1) One to four bone lesions; 2) One to four lung and/ or hepatic lesions; 3) Distant metastasis limited to ipsilateral cervical lymph nodes; 4) Distant metastasis limited to contralateral axillary lymph nodes
• Oligometastatic sites amenable to treatment with a local therapy modality, including surgical resection, stereotactic radiotherapy, or radiofrequency ablation
• Estrogen receptor-positive and/ or progesterone receptor-positive breast cancer
• Partial response or stable disease after at least six months of systemic therapy for breast cancer
• Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1
• Measurable or non-measuble disease according to Response Evaluation Criteria in Solid Tumors (RECIST) 1.1
• Life expectancy of at least 12 weeks
• For women in childbearing age, negative pregnancy test until 21 days before the date of study enrollment.
• Signed informed consent form
• Disposition and aptitude to fulfill the study protocol during the study duration

Exclusion Criteria:

• HER2-positive breast cancer
• Progressive disease during the last systemic treatment received for metastatic disease
• Previous local therapy for distant metastasis
• Current or previous history of severe diseases, such as clinically relevant heart failure, acute myocardium infarction in the last six months, chronic obstructive lung disease, HIV infection, chronic active hepatitis B or C infection, current serious uncontrolled infections or other severe diseases that may impact patients' expected survival)
• Current or previous history of other invasive malignancy within the last five years, excluding non-melanoma skin cancer