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

This is a prospective, randomized, monocentric, non-inferiority interventional clinical study comparing an Accelerated Partial Breast Irradiation (APBI) for the surgical bed of early-stage breast cancer patients with favorable histological subtypes, using stereotactic radiotherapy in a single dose of 15.5 Gy with Simultaneous Integrated Boost (SIB) to 21 Gy (study treatment, delivered in a single fraction) versus multifractionated radiotherapy of 30 Gy in 5 fractions (standard treatment, delivered in 5 fractions).

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 (\