Sue,
I believe the answer is usually "no", you can't have more RAD in the same area twice. However, a couple months ago, I remember seeing there were studies being done giving IMRT RAD to people that previously had other RAD. I don't know what the results have been so far and I don't even remember which website I saw it on. It was one of the main sites for finding clinical trials, I think. It might be worth it to ask the RAD doctor about it.
Rainbows & hugs,
Rosie