I am not an expert, but when my mother was undergoing radiation for tongue cancer she developed a fever a couple of times because the radiation had beaten up the inside of her mouth so badly that it became infected.She had a hard time using the saline rinse or anything else to clean her mouth because of the pain. I think it is referred to as thrush. She was put on antibiotics and it cleared up both times after about a week. I think this kind of thing is pretty common when a person is having radiation around their mouth.