Clinical Research Directory

Inclusion Criteria: * Postmenopausal (underwent bilateral oophorectomy or non-chemo induced amenorrhea for 12 or more months) female participant ≥60 years of age. * New diagnosis of invasive carcinoma of the breast (ductal, tubular or mucinous) with primary tumor ≤1 cm on microscopic exam with no evidence of nodal or distant metastatic disease. * Contralateral breast described as BI-RADS (Breast Imaging-Reporting And Data System) 1 or 2 in diagnostic imaging. * Negative axillary node involvement by sentinel node biopsy or axillary node dissection (pN0) or Clinically negative axillary node involvement (cN0) after mandatory ultrasound exploration without surgical exploration of the axilla if Breast Conservative Surgery (BCS) was performed. * Estrogen Receptor (ER) positive (≥ 50%) and Progesterone Receptor(PR) positive (\> 20%), Ki67 low (≤15%) and HER2 negative (IHC or In Situ Hybridization approach) according to ASCO (American Society of Clinical Oncology) criteria. * Histological grade scored on the invasive component, 1 or 2 if pT1a or grade 1 if pT1b. * Treated by mastectomy or BCS with microscopically clear resection margins defined as "no-ink on tumor" or ≥1 mm for invasive and non-invasive disease or no residual disease on re-excision. * If BCS was performed, participants must have received or have scheduled adjuvant local radiotherapy (RT) within 3 months after surgery * No indication of adjuvant chemotherapy. * The participant is willing and able to comply with the protocol for the duration of the study, including scheduled visits, treatment strategy, laboratory tests and other study procedures. * Participants must be affiliated with a Social Security System (or equivalent). * The participant must have signed a written informed consent form before any trial-specific procedures. When the participant is physically unable to give written consent, an impartial witness of her choice, independent from the investigator or the sponsor, can confirm the participant's consent in writing. Exclusion Criteria: * Have received any neo-adjuvant treatment, including hormone therapy and chemotherapy. * Have received or are eligible for adjuvant chemotherapy. * Absolute contraindication for RT in case of BCS or patients who were treated with partial breast irradiation. * Have been treated with mastectomy and with adjuvant radiotherapy received or scheduled. * Invasive lobular breast cancer. * Bilateral breast cancer or history of other invasive ipsi- or contralateral breast cancer. * Multifocal or multicentric disease. * Disease limited to microinvasion only (\< 1 mm). * Evidence of lymphovascular invasion. * History of non-breast cancer malignancies if not disease-free for \> 5 years and considered low risk of recurrence except for treated carcinoma in situ of the cervix, endometrium or colon, melanoma in situ and basal or squamous cell carcinoma of the skin. * Non-malignant severe disease associated with a life expectancy \< 10 years. * Women with BRCA1, BRCA2 or other high-risk breast cancer predisposing deleterious germline mutations. * Enrolled in another therapeutic study within 30 days prior to inclusion. * Unwilling or unable to comply with the medical follow-up required by the trial because of geographic, familial, social, or psychological reasons. * Persons deprived of their liberty or under protective custody or guardianship.