Surgery wait times during the COVID-19 pandemic were eventually reduced to pre-pandemic wait times. This study became irrelevant.

The standard treatment for early stage, estrogen-receptor positive breast cancer is lumpectomy and sentinel lymph node biopsy followed by 16-25 treatments of adjuvant whole-breast radiation therapy plus or minus hormone therapy. However, the COVID-19 pandemic has necessitated changes in the way breast cancer is treated in order to reduce contact between individuals, reduce spread of the novel coronavirus, and lessen the impact on health care resources. As elective surgeries are being cancelled, current pandemic guidelines recommend that patients be started on hormone treatment while waiting for surgeries to be re-instated. Only after this surgery occurs will patients receive radiation treatment, dramatically extending the time between diagnosis and end of treatment. Emergency pandemic guidelines in the UK and other countries recommend 5 fractions of pre-operative radiation therapy where appropriate. Based on previous work in the SIGNAL 1.0 and SIGNAL 2.0 clinical trials, the investigators are proposing treating patients with early stage breast cancers with one single fraction of stereotactic neoadjuvant radiation during the pre-operative waiting period. This will allow patients to complete radiation therapy upfront while reducing the number of patient visits to hospital. This will also allow investigators to evaluate the ability of single-fraction targeted radiation to induce a pathologic complete response.

Inclusion Criteria:

• Female sex
• Age ≥ 50 years old
• Postmenopausal
• Tumor size < 3cm on pre-treatment imaging
• Any grade of disease, estrogen receptor (ER) positive, HER2 negative
• Unicentric/unifocal disease
• Invasive ductal carcinoma or other favorable subtypes of epithelial breast malignancy (lobular, medullary, papillary, colloid, mucinous, or tubular) .
• Clinically node-negative (based upon pre-treatment physical examination and/or axillary ultrasound).
• Surgical expectation that a > 2mm margin can be obtained.
• Lesion is 1 cm or greater from the skin surface.
• Able to lie comfortably in the prone position with arms raised above the head for extended periods of time.

Exclusion Criteria:

• Male sex
• Under 50 years of age
• Previous RT to the same breast
• HER2 positive disease
• Evidence of suspicious diffuse microcalcifications in the breast prior to the start of radiation.
• Local metastatic spread into ipsilateral axilla and/or supraclavicular region and/or neck nodes and/or internal mammary nodes diagnosed on clinical examination or any imaging assessment (unless such sites can be confirmed as negative following biopsy)
• Distant metastases
• Involvement of contralateral axillary, supraclavicular, infraclavicular or internal mammary nodes (unless there is histologic confirmation that these nodes are negative)
• Prior non-hormonal therapy or radiation therapy for the current breast cancer
• Patients with Paget's disease of the nipple.
• Skin involvement, regardless of tumor size.
• Patients with a breast technically unsatisfactory for radiation therapy.
• Inability to lie prone with arms raised above head for extended periods of time.
• Patients not appropriate for BCS due to expectation of poor cosmetic result, even without RT
• Collagen vascular disease (particularly lupus, scleroderma, dermatomyositis)
• Inability or unwillingness to provide informed consent.
• Any other malignancy at any site (except non-melanomatous skin cancer) < 5 years prior to study enrollment
• Patients who are pregnant or lactating