My dad has Cis of the vocal cord and had the tumor [...]

My dad has Cis of the vocal cord and had the tumor removed and then started on radiation therapy. He was halfway through RT when he started having very bad side effects.He has severe sore throat, inability to swallow properly, pneumonia, sepsis and had to have a peg tube. Dr. said his reaction is highly unusual. He must be lacking an enzyme that helps with healing of radiated tissue. Anyone heard of this?