Study of Ribociclib Administered Concurrently With Postoperative Radiation Therapy in Patients With High-Risk, HR+/HER2- Breast Cancer

This study is testing whether ribociclib (a targeted cancer drug that blocks cancer cell growth) given at the same time as radiation therapy after breast cancer surgery improves outcomes in patients with high-risk hormone receptor-positive, HER2-negative breast cancer. **You may be eligible if...** - You are 18 years or older - You have hormone receptor-positive (ER+ or PR+) and HER2-negative breast cancer - You have had surgery that removed all visible cancer with clear margins - You have more than 3 lymph nodes involved, OR 1–3 nodes with a large tumor (T3) or grade 3 cancer - You are scheduled for radiation therapy after surgery **You may NOT be eligible if...** - Your cancer is HER2-positive - Your cancer is hormone receptor-negative - You have not had surgery or still have residual cancer after surgery - Your cancer does not meet the high-risk criteria listed above Talk to your doctor to see if this trial is right for you.