My husband has pretty much the same experience you do. Diagnosed about a year and a half ago. Went through Chemo (obtivo ) for about a year on and off and also with immunotherapy. A few months ago Radiation for a month every day. Cancer had spread to the right adrenal gland and underwent radiation for that as well for 5 treatments every other day. From the very beginning, heavy mucus would occur very soon after beginning to eat a meal. Quite random though, you never knew when the food would go down or not. Once it begins even to this day, he cannot get anything down including water! Sometimes if he's eating a sandwich, the bread seems to be the culprit. Just eating the contents will sometimes be okay but other times not that either. Nothing seems to work to change that. After a couple of hours and trying to cough it all up, he can finally eat something. It's extremely frustrating for him and for me to watch this take place. The oncologist just gave him a prescription for Naseau which of course is not the issue and does not help with phlegm. I offer one possible solution recommended to me by an older ENT. My husband will not use it because he doesn't think it will help. Perhaps you will be interested in trying it. I use it myself to help with my COPD. Alkalol, is sold at most drug stores and Amazon. It's a nasal wash, a mucus solvent. Use it in a netti pot or nasal douche, or gargle with equal parts Alkalol & hot water which will just warm it to gargle. Miracle in a bottle. It's excellent for a post-nasal drip and will loosen anything at the back of your throat. I hope this can offer you some relief. This link will explain more: Info@alkalolcompany.com