ERBB2 and breast cancer: In the recent PALOMA-2 and PALOMA-3 clinical trials, addition of palbociclib, a CDK4/6 inhibitor, to standard endocrine therapy significantly improved outcomes in treatment of ER-positive, HER2-negative advanced breast cancer (BC).1,2 However, the risks associated with concomitant combination of palbociclib and radiation therapy (RT) are unknown, and the majority of radiation and medical oncologists prefer to suspend palbociclib during RT.