Why is Cameron Diaz shave her head for the movie 'My Sister's Keeper'?

There is a scene in the trailer of 'My Sister's Keeper' where Sara (Cameron Diaz) is depicted shaving her head with a razor. What is up with that? By the way, I haven't the book as yet. Oh, do you'll think that if this movie is well-recieved by critics and various viewers, this could possibly result in some Academy Award nominations (after all, Abigail Breslin did recieve her first and only Oscar nomination to date when she was ten)? I believe anything is possible, don't you? What are the odds or what is the likelihood of that ever happening?