Radiotherapy (RT) is a fundamental pillar in the fight against cancer. In the case of breast cancer, RT has traditionally been applied as an adjuvant treatment following surgery. However, in recent years, there has been research into the potential benefit of administering RT prior to surgery in a neoadjuvant manner. Given that these studies have shown promising results, it is logical to think that certain patients could benefit from exclusive RT treatment, thereby avoiding surgery and its possible sequelae. This study is a phase I dose-escalation clinical trial with sequential assignment, open-label, in which five different doses (20Gy, 23Gy, 26Gy, 28Gy, and 30Gy) will be tested on the tumor using stereotactic body radiotherapy (SBRT) technique.

The aim of the study is to determine the maximum tolerated dose (MTD) based on acute toxicity, which will allow us to proceed with a phase II clinical trial to confirm efficacy and assess the late toxicity of the treatment. Secondary objectives include assessing the pathological response via core needle biopsy (CNB), radiological response, quality of life, and cosmesis. Additionally, molecular studies will be conducted on tissue and through liquid biopsy; pre- and post-treatment, we aim to identify mutations and predictors of response.

Inclusion Criteria:

• Over 18 years of age
• Female gender
• ECOG score 0-2
• Positive biopsy for invasive non-lobular breast tumor histology
• Tumor < 3 cm in diameter as measured by MRI or contrast mammography
• Tumor located ≥ 0.5 cm from the skin or rib cage, clinically or radiologically
• Positive estrogen and/or progesterone receptors
• HER2 negative
• No lymphovascular invasion in the biopsy
• Tumor must be clinically and radiologically N0 (no lymph node involvement). If a suspicious lymph node is seen, it must be biopsied and show a negative result
• Patient weight below 220 kg (weight limit for the treatment table)
• Ability to tolerate the supine position for 20 minutes during treatment
• Able to undergo MRI and/or contrast mammography
• Not pregnant; women of childbearing potential will undergo a pregnancy test to rule this out
• Contraindication to or refusal of surgical treatment, which must be explicitly documented in the medical record.

Exclusion Criteria:

• Under 18 years of age
• Male gender
• ECOG score 3-4
• Positive biopsy for invasive lobular carcinoma or ductal carcinoma in situ
• Tumor ≥ 3 cm in diameter as measured by MRI
• Tumor located < 0.5 cm from the skin or rib cage, clinically or radiologically
• Negative estrogen and progesterone receptors
• HER2 positive
• Presence of lymphovascular invasion in the biopsy
• Positive BRCA mutation
• Multicentric tumor
• Tumor with positive axillary lymph nodes
• Patients with metastatic disease
• History of previous breast cancer in the same breast
• Previous thoracic radiotherapy
• Pregnant or actively breastfeeding
• Unable to undergo MRI or contrast mammography
• Unable to tolerate the supine position
• Patient with breast implants
• Patients with pacemakers
• Scleroderma, lupus, or other connective tissue diseases with active flares
• Inability to understand and sign the informed consent