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

Inclusion criteria

• Female patient with a new histological diagnosis of clinical T1-3 N0 breast cancer (any tumour sub-type).
• Negative lymph node involvement at initial presentation, documented by imaging (US or MRI), fine needle aspiration (FNA) or core needle biopsy.
• Treated with a minimum of 8 weeks NAC, with patients with Her2+ disease receiving targeted anti-Her2+ therapy.
• Marker clip placed in the tumour bed prior to or during neoadjuvant chemotherapy when the tumour can still be identified.
• Treated by BCS with complete excision of the tumour bed and axillary staging surgery (either sentinel lymph node biopsy or axillary lymph node dissection).
• Final pathology demonstrating a pCR \[defined as absence of residual invasive and in-situ breast cancer within the breast or lymph nodes (ypT0N0)\].

Exclusion criteria

• Age less than 50 years.
• Inflammatory breast cancer or breast cancer invading the skin or chest wall (T4 disease).
• Multicentric disease (i.e., breast cancer involving more than one quadrant in the same breast).
• Prior history of ipsilateral or contralateral in-situ or invasive breast cancer. Patients with a history of lobular carcinoma in-situ (LCIS) are ineligible.
• Synchronous contralateral in-situ or invasive breast cancer.
• BRCA (breast cancer gene) 1 or 2 genetic mutation carrier, or other genetic mutation present associated with increased risk of breast cancer.
• Other previous non-breast malignancies except adequately treated non-melanoma skin cancers, in situ cancers or other cancers curatively treated with no evidence of disease for ≥ 5 years.
• Inability to complete entire course of neoadjuvant therapy (minimum of 8 weeks of treatment).
• Patients with HR+ (hormone receptor) disease who are not planned to have endocrine therapy initiated.
• Patients with Her2+ disease who have not received or are not planned to receive Her2 targeted therapy.
• ECOG (Eastern Cooperative Oncology Group) performance status \> 3.
• Inability to provide informed consent.