Janet Szabo's Chemo Toque seems to get rave reviews. Knitters who are chemo survivors all seem to insist on non natural fibers for chemo hats, even if they're ordinarily wool weenies of the most rabid sort. Some cottons also seem to be ok.

I've never had cancer, so I can't speak from experience. There are a lot of knitters who have survived cancer, so I listen to them .