This phase II study based on the Cyberknife Society study will evaluate the technical feasibility and acute toxicity of Partial Breast Irradiation (PBI) with the Cyberknife. It will evaluate quality of life (QOL) issues as they relate to treatment-related side effects, cosmetic result and patient convenience. It will evaluate outcome in terms of local control in the treated breast.

Radiosurgery is defined as the stereotactic delivery of ionizing radiation in 5 stages or less to a designated target with sub-millimeter accuracy. Radiosurgery in the context of this protocol will be given to the region of the tumor bed with 7 weeks of the lumpectomy and sent/axillary node sampling over the period of five to ten days using the Cyberknife (CK) or within the 7 weeks of the last chemotherapy treatment if given.

Inclusion Criteria:

• Patients with stage 0, I, or II (Tis, Ti, or T2 <=3cm & NO) histologically confirmed invasive non-lobular carcinoma or ductal carcinoma in situ of the breast with a lesion <= 3 cm, treated with wide excision.
• Negative inked histologic margins of excision or reexcision and clear of invasive tumor and DClS by at least 2mm
• Negative post-excision or post-reexcision mammography if cancer presented with malignancy-associated microcalcifications; no remaining suspicious microcalcifications in the breast before radiotherapy.
• No involved axillary lymph nodes by routine histologic examination (H&E) of sentinel node(s) or of nodes identified from axillary dissection.
• Enrollment and initiation of CyberKnife therapy within 42 days of last breast cancer surgery or last chemotherapy treatment.

Exclusion Criteria:

• Patients with invasive lobular carcinoma or nonepithelial breast malignancies such as sarcoma or lymphoma.
• Patients with multicentric carcinoma or with other clinically or radiographically suspicious areas in the ipsilateral breast unless confirmed to be negative for malignancy by biopsy.
• Patients who are pregnant.
• Patients who have any histologically confirmed positive axillary nodes.
• Patients with collagen vascular diseases, specifically systemic lupus erythematosis, scleroderma, or dermatomyositis with a creatine phosphokinase level above normal or with an active skin rash.