Radiation OmisSion in PAtients With CLinically Node Negative Breast Cancer Undergoing Lumpectomy

Inclusion Criteria:

1. Female patient with a new histological diagnosis of clinical T1-3 N0 breast cancer (any tumour sub-type).

2. Negative lymph node involvement at initial presentation, documented by imaging (USor MRI), fine needle aspiration (FNA) or core needle biopsy.

3. Treated with a minimum of 8 weeks NAC, with patients with Her2+ disease receivingtargeted anti-Her2+ therapy.

4. Marker clip placed in the tumour bed prior to or during neoadjuvant chemotherapywhen the tumour can still be identified.

5. Treated by BCS with complete excision of the tumour bed and axillary staging surgery (either sentinel lymph node biopsy or axillary lymph node dissection).

6. Final pathology demonstrating a pCR [defined as absence of residual invasive andin-situ breast cancer within the breast or lymph nodes (ypT0N0)].

Exclusion Criteria:

1. Age less than 50 years.

2. Inflammatory breast cancer or breast cancer invading the skin or chest wall (T4disease).

3. Multicentric disease (i.e., breast cancer involving more than one quadrant in thesame breast).

4. Prior history of ipsilateral or contralateral in-situ or invasive breast cancer.Patients with a history of lobular carcinoma in-situ (LCIS) are ineligible.

5. Synchronous contralateral in-situ or invasive breast cancer.

6. BRCA (breast cancer gene) 1 or 2 genetic mutation carrier, or other genetic mutationpresent associated with increased risk of breast cancer.

7. Other previous non-breast malignancies except adequately treated non-melanoma skincancers, in situ cancers or other cancers curatively treated with no evidence ofdisease for ≥ 5 years.

8. Inability to complete entire course of neoadjuvant therapy (minimum of 8 weeks oftreatment).

9. Patients with HR+ (hormone receptor) disease who are not planned to have endocrinetherapy initiated.

10. Patients with Her2+ disease who have not received or are not planned to receive Her2targeted therapy.

11. ECOG (Eastern Cooperative Oncology Group) performance status > 3.

12. Inability to provide informed consent.