If she understands that she will be getting medicine to make her feel better, you might explain to her that medicine sometimes has strange side effects. You can liken it to Harry Potter's magic which sometimes/always went askew in the hands of the students.
how much time is there?
You can also take her to the oncologist and let him explain or recommend a child psychologist who might be better informed about how to speak with children about such theings.
I don't believe you want to be the one to tell her, Mom. She will need someone who she can run to and in whose arms she can cry who is not a part of the medicine.
I hate to hear about these sort of things. I'm truly sorry. Let's hope that she comes out well on the other side of this.