Blowout.

If you’re talking about the carotid artery, it’s bad. Carotid blowout is the most feared complication of SBRT re-irradiation for head and neck cancer. In this large cohort that included multiple tumors encasing the carotid, the risk of blowout was ~5%. And there were no good dosimetric predictors. In other words, you can counsel your patients that the risk is low(ish) but the stakes are high.