I agree with Razzle that it might be medication induced, but there's also a possibility that it could be a reaction to a dye that would have been used if they did any prolonged fluoroscopic imaging during the surgery.

I did a web search for you and found many articles related to hair loss (alopecia) following brain surgery, and here's a link to those sites for you to browse through. I hope you find something useful there.
http://www.google.com.au/search?hl=e...meta=&aq=f&oq=