Would you hold abemaciclib during adjuvant chest wall/axillary radiation after mastectomy?  

I agree. There are some concerns that put cells into cell cycle arrest may also impact the benefits of radiation therapy. So it is recommended to hold abemaciclib during radiation therapy.

Adjuvant abemaciclib should be started after the completion of locoregional therapy as per the monarchE trial. While there is some evidence for CDK4/6 inhibition increasing the anti-tumor impact of radiation, there are also questions of increased radiation toxicity.