• Age older than 18

• Eastern Cooperative Oncology Group (ECOG) performance status 0-1

• Neurological function status 0, 1, 2

• Men and/or pre- or post-menopausal women with metastatic breast cancer with at least 2 intracranial untreated and measurable (≥ 5mm) metastases as visualized on brain MRI

• A diagnostic contrast enhanced MRI demonstrating at least 2 and no more than 10 measurable lesions in the brain, (≥5mm in size), performed within two weeks prior to treatment

• Maximum diameter of treated lesions should be \<4cm in size

• Have measurable disease based on RECIST 1.1. Lesions situated in a previously irradiated area are considered measurable if progression has been demonstrated in such lesions.

• Patient needs to be able to understand and demonstrate willingness to sign a written informed consent document

• Prior SRS is permitted, however the lesions targeted for treatment on trial need to be previously untreated by SRS

• Patients who have undergone prior subtotal resection are eligible providing that residual disease is \<4cm in maximum diameter: the cavity will be treated as - - - Continuing a concurrent use of hormonal therapy or anti-Her2 neu therapy is allowed, if the patient exhibits brain metastases progression during these treatments

• Enrolled patients should have a two-week washout period from last systemic treatment

• The participant (or legally acceptable representative if applicable) provides written informed consent for the trial.

• Contraception duration of 120 days

• Adequate bone marrow reserve and liver function