Unfortunately the muscle will be radiated along with the cancer. There’s no way to isolate it. I hate to post something that will be upsetting to you, but be prepared to have burns from the radiation. My husband convinced me to get a bidet that fit over the toilet. It was a lifesaver. Get one you won’t regret it. There are creams that have lidocaine in them. Get those too. Be prepared. There was a week when I couldn’t wear anything more than a robe. The skin becomes raw and there will be open wounds. Sorry to be so honest. I wish someone had prepared me. Doctors tell you all this after the fact.