The golden rule of writing is to write what you know. You don't know cancer, so perhaps you should come up with a different topic.
Cancer is not just one disease- there will never be just one cure. Many people DO survive cancer. Just because a teenager is diagnosed with brain cancer does not mean she is going to die. What kind of brain cancer are you writing about? There are very different outlooks for medulloblastoma and glioblastome multiforme, for example. I know several survivors of medullo, and none of GBM.
I just want to put this out there, because we see a lot of questions like yours, with people wanting to write about teens with cancer without having intimate knowledge of cancer themselves. I work with kids and teens who have cancer, and I recently had an interesting discussion with some of my teens. There's a popular young adult fiction book out now about a girl with cancer. Several of my patients read it, and read some other books about teens with cancer. They hated them. They felt that the books were so unrealistic that they were terrible. They were almost offended by these books- they felt that the authors were really just manipulating emotions with no real knowledge of what it's really like to go through cancer treatment as a teenager.
Work in pediatric oncology.