Most cancer patients that I've known prefer knitted hats. Crochet often has a looser fabric. The yarn is very important, too. It must be very soft. Wool or animal fiber of any yoe is not a good idea. The one I use most is Berroco Comfort. It's 50/50 acrylic and nylon.
I have a bunch bookmarked and will post them when I'm back on the computer.