Darlene,
There was a study by a Japanese group of 15 or so patients who had Gammaknife, then had surgery later after regrowth. It is hard to find a study with more cases, since regrowth doesn't happen that often. While they did report the dreaded "stickiness" in most cases, they found that about 25% had facial weakness or paralysis afterwards, not permanent, but the slow to recover kind. If you say the chance of regrowth is 4%, that works out to about a 1% chance of radiation - regrowth - surgery - facial paralysis. Not a bad risk.
I liked Kathleen's case (Kathleen5306), where she had surgery at House after regrowth, and they said it took 45 minutes longer to do the surgery, there were no problems, and they left the stickiest bits behind, realizing that they were sticky because they were radiated and dead.
As for the cause of regrowth, I can only surmise that Dr. Chang at Stanford, Dr. Medbery in Oklahoma, and other top radiation oncologists would dearly love to know what makes some of them regrow - either to identify the problem ones beforehand, improve the treatment, or prevent it happening afterwards. As far as I know, no one knows.
Steve