If your blood pressure is all over the map, even though you are sitting still, that is one issue some of us have. The more common issue is chronically low blood pressure and faintness as a result. Two contributing facts are scar tissue formation in the carotids (no good solution as stenting them does not have the same impact as stenosis from fats) and the destruction of the small nerve fibers near the carotid notch which are the sensors that tell your body what your blood pressure should be. This is a many years out of radiation issue, and highly dependent on number of grays and how heavily this area got nuked. The resulting syndrome is called barrio reflex disfunction. Most cardiologists have never even heard of it. But it is peer reviewed published on, and in the world of head and neck cancer patients, fairly well understood that it can happen. Again no workable fix that I personally have found.