When my son was 4 two important people we knew died of cancer. I used the 'folktale' approach to help him put it in context. It sounds like this may be way too simplistic for your DD, but I wonder if what she is looking for isn't a way to put this big scary topic into context.
Emotionally she is still 6 and may crave for an orderly universe that makes sense.
Here's my best shot:
All cells have ways to communicate with their neighbor-cells. This communication helps them be good team players. It tells them when to grow and when to wait, and how much the rest of the body needs their help in making whatever it makes to help the rest of the body. It's like how you have a little voice inside you that tells you how to act at school, when to ask a questions and when to wait to give someone else a chance. When to go first on the slide and when to let someone else go first. Everything that works in groups has to have ways to know how to work together. Cancer cells started as regular cells in your body, but their little voices, and their ears to hear communication from other cells got broken. This happens almost every day, and we don't have cancer because we have cells who have to job to look for these broken cells that grow too much and don't notice how they fit with the whole body. But once in a very very rare time, the cells who check make a mistake and the cancer cells get to big to be handled by your own body, so we get doctors to help take away these sad cells.
We also had discussions about evolution and the beauty of having DNA that is susceptible to change. The bad side is Cancer, but the good side is Evolution. I think I took the idea from Asimov's Foundation, of G-D reaching into the future and asking a representative Human 'do you want extra stable DNA that almost never breaks down and for evolution to take 1000 times longer than it did? or do you want it the way it is?' For all we know, the sun doesn't have enough fuel for humans to evolve quickly enough with super-stable DNA.
I hate losing people to Cancer, but I like the idea that flexibility is an important part of what makes humans human. It comforts me to think at least sometimes, things are the way they are because they have to be that way.
Also - look around at the rest of her life and try to reduce the stress in other areas if possible. Extra sensitive kids may not be able to comfortably handle what other kids her age handle, so be ready to think outside the box. Try to get a lot of physical contact, even playful wrestling in on a daily basis. It also helped my son to know that there were legal papers in place for who would take care of him should he be without DH and me. For a while I was strongly considering appointing an unofficial 'Educational Gaurdian' to help the official ones on matters related to giftedness and learning.
Love and More Love,
Grinity
