This is a prospective, single institution, phase I-II study of stereotactic body radiotherapy (SBRT) for unresected breast cancer. Twenty-four patients with advanced breast cancer who are not candidates for surgery (metastatic disease, unresectable, medically inoperable) or decline surgery will be enrolled in a prospective study using SBRT (4 fractions of radiation therapy over 12-15 days) as the primary treatment modality.

Inclusion Criteria:

Histologically confirmed diagnosis of invasive breast carcinoma of non-lobular histology.

Unifocal tumour Previously assessed by a breast surgeon and medical oncologist, with no surgery being recommended at time of enrollment.

Primary breast and axillary surgery not recommended/performed due to any of the following:

• distant metastatic disease
• unresectable T4 disease and/or
• medically inoperable and/or
• patient declined breast surgery Stages T1-T4, N1-N2, M0-M1 disease Ability to attend radiation planning and therapy, able to attend for follow-up care Able to provide written informed consent.

Exclusion Criteria:

Received chemotherapy within 3 weeks prior to breast SBRT. (Concurrent endocrine therapy is permitted) Multifocal or multicentric tumours. Ipsilateral supraclavicular or infraclavicular nodal disease >= 1cm size that is at the level of or superior to the brachial plexus Unable to meet the brachial plexus dose constraints despite compromising coverage of target volumes - as per the discretion of the treating physician Treatment target volume estimated to occupy > 50% of the ipsilateral whole breast volume.

Locally recurrent breast cancer, with previous radiotherapy to the breast, or previous lumpectomy or mastectomy. Ipsilateral breast implant. Serious non-malignant disease that precludes definitive surgical or radiation treatment (e.g.

scleroderma, systemic lupus erythematosus, cardiovascular/pulmonary/renal disease).

Women who are pregnant or lactating. Psychiatric or addictive disorders that preclude obtaining informed consent or adherence to protocol