I have this problem too - I have never had dandruff in my life but now it is like a snowstorm. I have found that kerastase oil treatments are helping. I am currently using Elixir Ultime Rose Millenaire, and also the elixir ultime shampoo. It isn't cheap but it does seem to have made a difference.
Try this link, if the Mods allow me to post it. http://www.nhs.uk/ipgmedia/national/my%20new%20hair/Assets/MyNewHair-haircareaftercancertreatment8pa...
Page 22 deals with the scalp. Hope it is helpful.
I finished chemo in June 2014, but still had heraceptin until February 2015. The problem I am having is now that my hair has grown back is dry scalp. I have consulted several hair dressers with solutions to this problem. Nothing to date has worked. I have tried coconut oil on my scalp at night. I would apply all over my head and sleep with it in and then would shampoo it out in the morning. I tried this for 2 months with no luck. I have tried 10 different shampoos with no luck, including organic shampoos. My dry scalp is so severe that I do not wear anything black because it looks like my shoulders are completely covered in white. Very disgusting to look at and even more difficult to explain. Does anyone have any suggestions?