The suggested wait period is 21 days after an extraction before starting radiation treatment, but some doctors say 14 days is ok. Anything sooner my involve being on prophylactic antibiotics. If you still have an infection, I would think that needs to be cleared before starting Chemoradiation, which can compromise your immune system further, go systemic.

As mentioned, you should have a dental tray, prescription fluoride, dental rinse like chlorhexadyne, and some are fitted with a latex dental guard to prevent radiation scatter from the dental fillings, which can help with mucocitis.