Three years ago, Sarah, age 54, completed standard chemotherapy and radiation treatment for stage II, ER- and PR-positive, HER2-negative invasive breast cancer. A recent computed tomography scan, ordered to evaluate persistent hip pain, revealed bone lesions, and a biopsy and positron-emission tomography scan confirmed bone-only metastatic breast cancer. A CDK4/6 inhibitor, ribociclib, was added to the letrozole she was already taking.