What is the optimal endocrine therapy for postmenopausal women with hormone receptor-positive early breast cancer?

A 56-year-old postmenopausal woman with a recent diagnosis of breast cancer was referred to discuss adjuvant therapy. Annual screening mammogram demonstrated a suspicious mass in the left breast. Ultrasound-guided core needle biopsy revealed an infiltrating ductal carcinoma that was estrogen receptor (ER) positive and progesterone receptor (PR) negative and lacked amplification of human epidermal growth factor receptor 2 (HER2; ie, HER2 negative). She underwent excision and sentinel node evaluation. Pathology demonstrated a 1.9-cm grade 2 invasive cancer without lymphatic vascular invasion; clean margins were obtained, and both sentinel nodes were free of cancer. The 21-gene recurrence score was 16. She has a body mass index (BMI) of 28.5 but is otherwise healthy; levothyroxine is the only prescription medication she takes. She experienced vaginal spotting 2 years earlier because of an endometrial polyp, which was resected. She exercises regularly and takes a calcium supplement with vitamin D. Bone density study performed 6 months earlier was normal other than mild osteopenia in the femoral neck (T score, -1.3). Radiation therapy is planned.