I had radiation and chemo 24 years ago for Hodgkins, then again 4 years ago for head and neck, (tonsil) lost 50 each time. Radiation will give you sores in the mouth and throat so it's hard to eat/swallow. I started eating everything in sight when I was diagnosed, even before starting treatment, I knew it was coming so I started trying to pack on weight. Good thing too, as I did loose a bunch. Just try to eat as much as you can. My dr gave me a couple of mouthwashes to use to help with the sores, and to numb it a littel so I could swallow. Its miserable, but it will only last a while, then you will get better. I also lost my voice for a month, could only whisper. Hight calorie/hight nutrient drinks help too.
Hope it goes easy and quick for you!