Steve,
We were told by our former ENT who was NOT affiliated with a cancer center that radiation could not be repeated.
When my husband had the recurrence, we opted to pursue treatment at a cancer center. They indeed radiated again, but we were told that this will be the last time he can be radiated on that side of his neck.
There have been some problems with cellulitis and infections of the soft tissues since he was treated, but so far, that has been a small price to pay for what really looked like an initial treatment protocol. The radiation was administered differently the second time...rather than daily, it was done 2 times a day six hours apart...and the field was more targeted and less deep.
I suspect each case is different, but from our interaction with "regular," non-cancer center affiliated ENTs, the ENTS and other docs at the centers are better informed of various treatment options, including reirradiation.
Good luck. Let us know how things turn out for you.
Loretta