Sofia Vassilieva shaved off her hair and eyebrows to play Kate Fitzgerald. She described it as the least she could do to understand Kate's pain. She was filming this movie and Medium (2005), so she wore a wig for the show.
Although Cameron Diaz's character in the movie shaves her head, Diaz does not do so in real life. Sofia Vassilieva, who played Diaz's cancer-stricken daughter, did, however, shave her head in both the movie and in real life.
Real-life sisters Elle Fanning and Dakota Fanning were originally set to play Anna and Kate Fitzgerald. Elle bowed out after Dakota reportedly refused to shave her head, as required for the part.
Cameron Diaz wore a bald cap instead of shaving off her own hair. This was more for practical reasons as Diaz only needed to be bald for just two scenes.