Have any of your friends or relatives that your dd is close to dealt with cancer? I'm just curious if her anxiety over cancer is due to knowledge or partly due to not being able to control what is happening with a loved one? Second question, is she an anxious child in general or is this anxiety something new and completely fueled simply by learning about cancer?
If it was me, I'd probably try to calm her anxieties but also redirect so that her attention can be focused on something less stressful to her for awhile. Re cancer specifically, I might also look for some examples of people for her to learn about - cancer researchers who've made strides in understanding cancer - so she can see that in spite of how overwhelming it is, we've made tremendous progress in understanding it and finding new ways to treat it over the past 50 years. Then maybe move on into other types of medical research/advances or advances in other types of technology or anything that gently leads away from hyper-focusing on cancer.
polarbear
