I had the lump in my neck appearing during radistion, and was concerned too. Dr said the same thing, it was from radiation, I called it a turkey neck, and took about a year to subside, but do show it to your doctor. The the thick ropey mucus is from the radiation effect on the salivary glands, which make thick and thin saliva, some a combination, or more of one or the other, but the parotid gland mainly produces thin saliva, and radiation only destroys thin saliva, not thick, so is the reason for the thick secretions. that may subside 3 weeks after treatment. As mentioned, rinsing helps, soda, brushing the teeth, tongue, maybd with a soft or a baby tooth brush. Mucinex, robotusem help with the mucus, rinsing with papaya juice or pineapple juice, and another is meat tenderizer diluted with water or mix with the juices. Tea also helps break up mucus. Sleeping on an incline also helps prevent it from backing up.

Good luck.