Jeff,
At this point I reallly wouldn't worry about any food that might "cause" cancer. After all, its very well documented that radiation can cause cancer, but a lot of us would not be here due to cancer if not for radiation treatment.
I would recommend quinoa, a seed that cooks like rice, goes down easier because it's smaller, and contains complete proteins. Fruit ices and juice bars can be soothing if she can tolerate the fruit. Pureed soups, like split pea and bean are a great source of protein too. And smoothie are wonderful. I used yogurt in mine, but there are lots of recipes available online that use other ingredients.
I was on a feeding tube, using a fiberless formula that can also be taken by mouth, so that might be something to look into.
Susan