Deescalation of Endocrine Therapy Duration in Women With HR+ HER2- Breast Cancer at Very Low Risk

Inclusion Criteria:

1. Postmenopausal women: Postmenopausal status is defined by any of the following:

• Prior bilateral oophorectomy

• Age ≥60 years

• Age >50 and <60 years and amenorrheic for at least 12 months, andfollicle-stimulating hormone (FSH) and estradiol in the postmenopausal range

2. Eastern Cooperative Oncology Group (ECOG) performance status 0-1

3. Women with histologically proven invasive unilateral breast cancer Note: In case ofa multifocal invasive tumor, all lesions (maximum 3 infiltrating lesions allowed)must be of identical phenotype and low biological risk

4. M0: Not clinically nor radiologically detectable metastases at time of inclusion

5. Primary tumor completely resected and adequate axillary surgery performed, accordingto current standards

6. IHC expression of the estrogen receptor and/or progesterone receptor ≥50%

7. HER2 negative according to ASCO criteria in immunohistochemistry and/or genomicanalysis (HER2 negativity is defined as IHC 0-1+, or [IHC 2+ and FISH or CISHnonamplified])

8. No indication of adjuvant chemotherapy

9. pT1 (tumor ≤20 mm), pN0, Grade 1 or Grade 2 OR pT2 (tumor ≤30 mm) and pN0, Grade 1or Grade 2 Note 1: patient with Grade 2 pT2pN0 tumor must be aged under 70 years of age andshould receive a genomic test as part of standard care (RIHN reimbursement)

10. Patient considered has having a luminal A ultralow risk of metastatic recurrence (i.e.less than 5% risk of metastatic relapse at 10 years) according to MammaPrint®and Blueprint® tests. Note 1: To be eligible, MammaPrint index score should be > +0.355

11. Patients eligible to receive or have recently started (with a maximum of 4 months ofadjuvant hormone therapy prior to enrollment) an adjuvant hormone therapy (letrozole, anastrozole, or exemestane)

12. Patient is willing and able to comply with the protocol for the duration of thestudy including scheduled visits, treatment plan, laboratory tests and other studyprocedures

13. Patients must be affiliated to a Social Security System (or equivalent)

14. Patient must have signed a written informed consent form prior to any trial specificprocedures. When the patient is physically unable to give their written consent, atrusted person of their choice, independent from the investigator or the sponsor,can confirm in writing the patient's consent.

Exclusion Criteria:

1. Patients who received a neo-adjuvant hormone therapy, a neo-adjuvant or adjuvantchemotherapy or preoperative medical treatment

2. Any local or regional recurrence or metastatic disease

3. Non-invasive carcinoma

4. Bilateral breast cancer (except in case of contralateral DCIS), or history of otherinvasive ipsi- or contralateral breast cancer

5. Patients with a history of another malignancy, except for properly treated cervicalcarcinoma in situ, and non-melanoma cancer of the skin

6. Women with high-risk breast cancer predisposing deleterious germline mutations

7. Contra-indications to the administration of anti-aromatase inhibitors

8. Patients enrolled in another therapeutic study within 30 days prior to inclusion

9. Patients with any other disease or illness, which requires hospitalization or isincompatible with the trial treatment

10. Patients unwilling or unable to comply with trial obligations for geographic,social, physical or psychological reasons, or who are unable to understand thepurpose and procedures of the trial

11. Persons deprived of their liberty or under protective custody or guardianship