• Histologically or cytologically confirmed newly diagnosed clinical stage II or III (by AJCC 8th edition - at least T2, any N, M0 or if N1+ then any T) ER-positive (ER \> 10%), any PR, and HER2-negative breast cancer with complete surgical excision of the breast cancer after neoadjuvant therapy as the treatment goal.

‣ HER2 negative must be assessed by FISH or IHC staining 0 or 1+ according to ASCO/CAP guidelines.

• A palpable mass is not required; however, tumor size must be either:

‣ ≥2 cm in one dimension by clinical or radiographic examination (WHO criteria), if clinically axillary lymph node negative OR

⁃ Measureable (≥10 mm) by modified RECIST v1.1 for breast MRI (see Section 9.0), if histologically confirmed resectable locoregional nodal involvement.

• ECOG performance status 0 or 1.

• Eligible to receive neoadjuvant aromatase inhibitor, as per treating physician.

• Eligible to receive neoadjuvant standard of care anthracycline- and/or taxane-based chemotherapy regimen, as per treating physician.

• Able to tolerate breast MRI with intravenous contrast administration. Must be able to complete the applicable MRI screening evaluation form.

• Adequate bone marrow and organ function, as determined by the treating physician.

• Known history of hepatitis C virus (HCV) infection is permissible provided the patient has been treated and cured.

• At least 18 years of age.

• Postmenopausal status, defined as one of the following:

‣ Age ≥ 60 years

⁃ Age \< 60 with intact uterus and amenorrhea for 12 consecutive months or more

⁃ Status post bilateral oophorectomy, total hysterectomy

• Ability to understand and willingness to sign an IRB approved written informed consent document (or that of legally authorized representative, if applicable), and willing and able to comply with scheduled visits and treatment schedule.