They are definitely not the same thing at all. I had to use a mouth guard for my radiation, and it was not a choice. They want to be sure your mouth stays in exactly the same position for every single radiation treatment, so that the targeted radiation hits the correct spots. It hurt SO bad near the end to force that stupid thing in my mouth that they had to up my painkillers just for that. (And besides the mouth guard there was the head thing that holds you still, too ... they do not want to be frying the wrong parts ... they take it seriously, or they should be.)

If your radiologist or oncologist are not having you use a mouth guard, I'd ask why. It may be that your type of treatment doesn't require it, or it may be that it got overlooked while setting everything else up. Can't hurt to be sure.

Oh, and Christine mentioned fillings ... THAT is a whole different problem, and nobody told me about that one either, till near the end when the damage was done. If you have fillings, they will cause radiation scatter and more pain and sores. They can give you a sort of wax to put over your teeth with fillings to help with the scatter (not sure how it works, or why it should, but they will offer it) but please ask for it near the beginning, before the damage is done! The radiation techs were the ones who let me in on that tip and were upset no one had told me about fillings being an issue before that point ... and we were I don't even know how many weeks into my treatment at that point. Gah.

Not trying to be a downer, just trying to share tips. Nobody should have to go through unnecessary pain!