I tend to obsess about things that worry me as well - pretty sure it's a gifted thing. I don't know what this looks like in a six year old, so take what I'm about to say with a grain of salt.
I find that if I immerse myself in whatever is bothering me and find out as much information as I can, it tends to soothe my fears. My current fixation is airplanes (terrified of them). The other day my husband commented "this is what you're obsessed about now?" LOL yup.
For me the bottom line is
facts are far less scary than what my vivid imagination can conjur up. If I face my fears directly and immerse myself in them, they have far less power over me. (This may be the case with your daughter as well?)
I love the idea of exploring medical research - this could be very empowering for your daughter. Maybe you can also teach her about the power of the human immune system and all of its defenses against cancer. Being an anxiety ridden, obsessive person myself, I'm not sure you'll be able to get her attention completely away from cancer (but who knows - it's worth a try).
If you can't distract her from it, you can sift through cancer stats (yourself first) and share with her only the empowering ones. For example, there are over 100 different kinds of cancer, and none of them have 100% mortality rate: This means that EVERY single type of cancer has survivors. (I myself had cancer 13 years ago - it was cured and has never come back
) Some cancers have a lower mortality rate than common conditions such as Asthma. Some people live for 30-40 years with low grade cancer that never kills them (like a friend of mine)... it just becomes a chronic condition that needs monitoring and occassional intervention (like the periodic removal of slow growing tumors). I swear, having cancer was one of the best things that ever happened to me because I'm no longer afraid of it.
(I don't mean to take away from the suffering of those who have lost loved ones to the aggressive forms of cancer - it can be devastating - it's just that there are other, less terrifying sides to cancer that you can talk to your daughter about).
Good luck - I hope she makes it in to Davidson
