Hi Cindy,
I am sorry to hear that your mother is experiencing vertigo. I have had episodes of vertigo since the end of my radiation in July of 2009. Not to the point of being debilitating, but not very pleasant. I reached out to my Family MD and she suggested the Epley Maneuver to manage my bouts of vertigo. It might be helpful for you and your mother to speak with her MD about what is going on with her. FYI, my RO didn't really have much to offer by way of explanation or suggestions either but I find the Eply Maneuver works quite well for me when I am having episodic vertigo.
It is really important to get through the full course of radiation for your mother to have the best chance she can at survival. Hang in there, Cindy. Best of Luck and let us know how it goes! Take Care.