Single-dose Preoperative Partial Breast Irradiation in Low-risk Breast Cancer Patients

Inclusion Criteria: * WHO performance scale ≤2. * Females at least 50 years of age with unifocal cT1N0 breast cancer on mammography, ultrasound and MRI. * Patients with an indication for chemotherapy or HER2-targeted therapy according to Dutch National Oncoline Guidelines or own hospital protocols are not eligible. Patients with an indication for endocrine therapy are eligible. * Tumor size as assessed on MRI. * On tumor biopsy: * Bloom-Richardson grade 1 or 2. * Non-lobular invasive histological type carcinoma. * LCIS or (non-extensive) DCIS is accepted. * ER positive tumor receptor. * HER2 negative tumor. * Tumor-negative sentinel node (excluding isolated tumor cells). * Adequate communication and understanding skills of the Dutch language. Exclusion Criteria: * Legal incapacity. * BRCA1, BRCA2 or CHEK2 gene mutation. * Distant metastasis. * Previous history of breast cancer or DCIS. * Other type of malignancy within 5 years before breast cancer diagnosis. Patients with adequately treated malignancy longer than 5 years before breast cancer diagnosis are eligible for inclusion. * For adequately treated carcinoma in situ of the cervix or basal cell carcinoma of the skin no specific time span to breast cancer diagnosis is required for inclusion. * Collagen synthesis disease. * Signs of extensive DCIS component on histological biopsy or on imaging (e.g. no extensive calcifications on mammography). * Invasive lobular carcinoma. * MRI absolute contraindications as defined by the Department of Radiology. * Nodal involvement with cytological or histological confirmation. * Indication for treatment with (neo-)adjuvant chemotherapy. * Non-feasible dosimetric RT plan.