Barbara,
Our previously radiated mouths just do not respond the same way unradiated mouths do. It sounds to me like you and your husband, and perhaps the dentist(s) are not certain if it is the extraction or the root canal that is causing the problem. I guess if it were me I would consult with the doc you most respect/trust, and ask them what you should do, although I don't know how much your oncologist likes to mess with dental matters. Still would not hurt to ask them for guidance. And maybe the antibiotic will finally prevail, it happens.
Anne

In rereading your previous post, I saw that you did consult the oncologist & the nurse though he would not want to mess with dentistry. So sorry, I got confused with all the doctors--I have the same situation in my life now! So seems like your options are go somewhere else in dentistry, getting another opinion (like MD Anderson) or stay the course where you are, at least for a little while longer. Sorry for the confusion.