RecruitingPhase 1Drug trialStudy of Ribociclib Administered Concurrently With Postoperative Radiation Therapy in Patients With High-Risk, HR+/HER2- Breast CancerThe purpose of this research study is to determine the safety, tolerability and dose of Ribociclib when combined with adjuvant radiation in women with high-risk ER+ breast cancer. Once enrolled on study, patients will begin treatment with Ribociclib 400 mg daily at the same time as they initiate standard of care adjuvant radiation therapy- 50 Gy in 25 fractions or 42.56 Gy in 16 fractions +/- 10 Gy boost including comprehensive nodal. Paitents will continue treatment with Ribociclib for up to 6 weeks.