Partial Breast Irradiation Without Sentinel Lymph Node Biopsy in Low-Risk Early Breast Cancer

Inclusion Criteria: * Female ≥40 years with histologically confirmed unilateral invasive breast cancer. * No suspicious nodal metastasis on preoperative axillary ultrasound (cN0). * Single lesion in the affected breast on preoperative breast ultrasound and mammography. * Treated with breast-conserving surgery with pathologically negative margins for the invasive tumor. * Sentinel lymph node biopsy not performed at surgery (pNx). * Maximum diameter of invasive tumor ≤2 cm on final pathology (pT1). * Histologic grade 1 or 2. * Signed informed consent prior to enrollment. Exclusion Criteria: * History of malignancy other than breast cancer within 5 years (except adequately treated non-melanoma skin cancer, or carcinoma in situ excluding breast carcinoma in situ). * Preoperative diagnosis of carcinoma in situ without axillary nodal sampling, subsequently diagnosed as invasive breast cancer on final pathology with SLNB omitted. * Lymphovascular invasion present (LVI+). * Multifocal or multicentric tumor in the same breast confirmed radiologically or pathologically. * Bilateral or inflammatory breast cancer. * Prior radiotherapy to the breast or thorax. * Confirmed pathogenic or likely pathogenic variant in a hereditary breast cancer gene (including BRCA1/2). * Recurrent breast cancer.