Such a bad smell is often caused by a condition called sloughing,an unpleasant part of the post rad healing process which is when necrotic or dead skin peels away from the lining of the mouth or throat,and will be in the globs of mucous expectorated when you cough .Hopefully it reveals pink healthy tissue underneath which shows that healing of rad burns is under way.This can also be made worse by the presence of thrush which requires prescription medication,