This is a dose escalation study involving stage IV breast cancer patients not progressing after 6 months of first line systemic treatment. Potential advantages of stereotactic body radiotherapy (SBRT) in treating breast primary tumor in metastatic breast cancer are:

• radio-biological advantage of a short highly effective treatment schedule
• possibility of preventing lesions to become symptomatic
• possibility of continuing systemic treatment without interruption

Inclusion Criteria:

• Age: 18 years or older
• Histologically confirmed diagnosis of invasive breast carcinoma (biological immunohistochemical profile: luminal and/or human epidermal growth factor receptor 2 positive)
• Distant metastatic disease not progressing after 6 months of systemic therapy
• Zubrod Performance status of 0-1
• Unifocal tumour < 5 cm tumor size
• Tumor CT or 18-labeled fluorodeoxyglucose positron emission tomography (5FDG-PET) detectable
• No surgery being recommended at time of enrollment.

Exclusion Criteria:

• prior radiotherapy
• active systemic lupus erythematosus, or any history of scleroderma, dermatomyositis with active rash
• Women who are pregnant or lactating.
• Psychiatric or addictive disorders that preclude obtaining informed consent or adherence to protocol