Steve
OCF pages on side effects of radiation are here
OCF side effects radiation
For a more general overview of radiation
OCF radiation
Basically the radiation will burn the heck out of your mouth, and you will have increasing difficulty swallowing. Some people get a PEG or feeding tube, others do not and just switch to Ensure Plus or a liquid diet for the duration. Nausea varies, and the first week is a breeze, the second week not so much, and by the third week you are really feeling it which continues for about a month after the TX is done.
So your dietary concern is to get at least 2,000 to 3,000 calories a day and not vomit it up. You can search this forum for posts on any specific subject with the search box on the upper right.
Finally, radiation TX is very doable, heck I even did it twice.
Charm