Single-fraction APBI for Early-stage Breast Cancer With Favorable Histological Subtypes (Breast-1F)

Inclusion Criteria: 1. Histological diagnosis of breast cancer 2. Patients aged ≥40 years 3. Signed informed consent 4. Stage pTis-T2, up to 3 cm in the greatest diameter 5. Luminal A and Luminal B HER2-negative histological subtypes 6. Negative surgical margins (≥ 0.2 cm) 7. Negative lymph nodes at sentinel lymph node biopsy, or alternatively with imaging tests (Positron Emission Tomography (PET)/Computed Tomography (CT) and/or axillary ultrasound, and/or breast Magnetic Resonance Imaging (MRI) with and without contrast), or, in rare cases, where still performed, with axillary dissection 8. Clinical M0 (PET/CT and/or bone scintigraphy and/or abdomen-pelvis CT with and without contrast in suspected patients), within the previous 3 months. 9. Performance Status (PS) Eastern Cooperative Oncology Group (ECOG) ≤2 10. No prior thoracic radiotherapy 11. Fertile women using contraception methods initiated during oncological treatment. Exclusion Criteria: 1. Patients aged \< 40 years 2. Tumors \> 3 cm 3. Multicentric tumors 4. Positive or close surgical margins (\<0.2 cm) 5. Lobular carcinoma 6. Negative hormone receptors 7. HER2-positive 8. BReast CAncer (BRCA) genes BRCA1 and/or BRCA2 positive (only if known) 9. Severe systemic diseases 10. Psychiatric or other disorders that may prevent the patient from signing informed consent 11. Previous invasive cancer, except for skin cancer (excluding melanoma) unless the patient has been disease-free for at least 3 years (e.g., carcinoma in situ of the oral cavity or bladder) 12. Collagen or autoimmune diseases (systemic lupus erythematosus, rheumatoid arthritis, scleroderma, Sjogren's syndrome) 13. Lymph node disease (N1) 14. Evidence of distant metastasis (M1) 15. Contraindication to systemic treatment 16. Pregnant women 17. Non-compliance with the dose limits established in the treatment plan