My mother in law has been going thru cancer treatment for one and a half years now. I did not know how to knit at the time, but I did crochet her several hats out of a microfiber yarn, who's exact name I can't remember right now (bought it at Michaels, and it comes in several colors). She absolutely loves these hats and they have only gotten softer when washed.
I then found those knifty knitter looms and started making her hats on that. I used cha-cha and fun fur. She also liked them, but likes the microfiber onces best of all. The cha cha and fun fur hats I thought were cute because they make her look like she has hair again!
The most important thing to remember if you make these caps is that they have no seams as the skin of a chemo patient gets very sensitive from all the chemicals they are receiving and the small ridge from the seam seems to irritate them.
Since I have learned to knit, I have not made her any hats, mainly because she let me know she has more than enough!
Good luck, it's a worthy project!